feat(PRT): add MF6 PRT support utilities

* to_coords() method for particle data classes
* to_mp7_[pathlines/endpoints] conversion fns
* support PRT pathline data in plot routines
* add PRT tutorial and example notebook

.docs/Notebooks/mf6_prt_tutorial01.py

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+# ---
+# jupyter:
+#   jupytext:
+#     notebook_metadata_filter: metadata
+#     text_representation:
+#       extension: .py
+#       format_name: light
+#       format_version: '1.5'
+#       jupytext_version: 1.15.0
+#   kernelspec:
+#     display_name: Python 3
+#     language: python
+#     name: python3
+#   metadata:
+#     section: mf6
+# ---
+
+# # MODFLOW 6 PRT (particle-tracking) tutorial
+
+# This tutorial runs a GWF model then a PRT model
+# in separate simulations via flow model interface.
+#
+# The grid is a 10x10 square with a single layer,
+# the same flow system shown in the FloPy readme.
+#
+# Particles are released from the top left cell.
+#
+# ## Initial setup
+#
+# First, import dependencies, set up a temporary workspace, and define some filenames and model parameters.
+
+
+import os
+from pathlib import Path
+from tempfile import TemporaryDirectory
+
+import flopy
+import matplotlib.cm as cm
+import matplotlib.pyplot as plt
+import numpy as np
+import pandas as pd
+from flopy.utils import PathlineFile
+from flopy.utils.binaryfile import HeadFile
+
+# workspace
+temp_dir = TemporaryDirectory()
+ws = Path(temp_dir.name)
+
+# model names
+name = "prtfmi01"
+gwfname = f"{name}_gwf"
+prtname = f"{name}_prt"
+mp7name = f"{name}_mp7"
+
+# output file names
+gwf_budget_file = f"{gwfname}.bud"
+gwf_head_file = f"{gwfname}.hds"
+prt_track_file = f"{prtname}.trk"
+prt_track_csv_file = f"{prtname}.trk.csv"
+mp7_pathline_file = f"{mp7name}.mppth"
+
+# model info
+nlay = 1
+nrow = 10
+ncol = 10
+top = 1.0
+botm = [0.0]
+nper = 1
+perlen = 1.0
+nstp = 1
+tsmult = 1.0
+porosity = 0.1
+
+# Define release points. We will release three particles from the top left cell of the model.
+
+# release points
+releasepts = [
+    # particle index, k, i, j, x, y, z
+    # (0-based indexing converted to 1-based for mf6 by flopy)
+    (0, 0, 0, 0, 0.25, 9.25, 0.5),
+    (1, 0, 0, 0, 0.5, 9.5, 0.5),
+    (2, 0, 0, 0, 0.75, 9.75, 0.5),
+]
+
+# Particle-tracking models require a flow solution to solve for particle motion — PRT models may either run side-by-side with a GWF model with an exchange, or may consume the output of a previously run GWF model via flow model interface.
+#
+# In this tutorial we do the latter. First we define a GWF model and simulation, then a separate PRT model/simulation.
+#
+# Define a function to build a MODFLOW 6 GWF model. 
+
+def build_gwf_sim(ws, mf6):
+    # create simulation
+    sim = flopy.mf6.MFSimulation(
+        sim_name=name,
+        exe_name=mf6,
+        version="mf6",
+        sim_ws=ws,
+    )
+
+    # create tdis package
+    flopy.mf6.modflow.mftdis.ModflowTdis(
+        sim,
+        pname="tdis",
+        time_units="DAYS",
+        nper=nper,
+        perioddata=[(perlen, nstp, tsmult)],
+    )
+
+    # create gwf model
+    gwf = flopy.mf6.ModflowGwf(sim, modelname=gwfname, save_flows=True)
+
+    # create gwf discretization
+    flopy.mf6.modflow.mfgwfdis.ModflowGwfdis(
+        gwf,
+        pname="dis",
+        nlay=nlay,
+        nrow=nrow,
+        ncol=ncol,
+    )
+
+    # create gwf initial conditions package
+    flopy.mf6.modflow.mfgwfic.ModflowGwfic(gwf, pname="ic")
+
+    # create gwf node property flow package
+    flopy.mf6.modflow.mfgwfnpf.ModflowGwfnpf(
+        gwf,
+        pname="npf",
+        save_saturation=True,
+        save_specific_discharge=True,
+    )
+
+    # create gwf chd package
+    spd = {
+        0: [[(0, 0, 0), 1.0, 1.0], [(0, 9, 9), 0.0, 0.0]],
+        1: [[(0, 0, 0), 0.0, 0.0], [(0, 9, 9), 1.0, 2.0]],
+    }
+    chd = flopy.mf6.ModflowGwfchd(
+        gwf,
+        pname="CHD-1",
+        stress_period_data=spd,
+        auxiliary=["concentration"],
+    )
+
+    # create gwf output control package
+    oc = flopy.mf6.ModflowGwfoc(
+        gwf,
+        budget_filerecord=gwf_budget_file,
+        head_filerecord=gwf_head_file,
+        saverecord=[("HEAD", "ALL"), ("BUDGET", "ALL")],
+    )
+
+    # create iterative model solution for gwf model
+    ims = flopy.mf6.ModflowIms(sim)
+
+    return sim
+
+
+# Define the PRT simulation.
+
+def build_prt_sim(ws, mf6):
+    # create simulation
+    sim = flopy.mf6.MFSimulation(
+        sim_name=name,
+        exe_name=mf6,
+        version="mf6",
+        sim_ws=ws,
+    )
+
+    # create tdis package
+    flopy.mf6.modflow.mftdis.ModflowTdis(
+        sim,
+        pname="tdis",
+        time_units="DAYS",
+        nper=nper,
+        perioddata=[(perlen, nstp, tsmult)],
+    )
+
+    # create prt model
+    prt = flopy.mf6.ModflowPrt(sim, modelname=prtname)
+
+    # create prt discretization
+    flopy.mf6.modflow.mfgwfdis.ModflowGwfdis(
+        prt,
+        pname="dis",
+        nlay=nlay,
+        nrow=nrow,
+        ncol=ncol,
+    )
+
+    # create mip package
+    flopy.mf6.ModflowPrtmip(prt, pname="mip", porosity=porosity)
+
+    # create prp package
+    flopy.mf6.ModflowPrtprp(
+        prt,
+        pname="prp1",
+        filename=f"{prtname}_1.prp",
+        nreleasepts=len(releasepts),
+        packagedata=releasepts,
+        perioddata={0: ["FIRST"]},
+    )
+
+    # create output control package
+    flopy.mf6.ModflowPrtoc(
+        prt,
+        pname="oc",
+        track_filerecord=[prt_track_file],
+        trackcsv_filerecord=[prt_track_csv_file],
+    )
+
+    # create the flow model interface
+    flopy.mf6.ModflowPrtfmi(
+        prt,
+        packagedata=[
+            ("GWFHEAD", gwf_head_file),
+            ("GWFBUDGET", gwf_budget_file),
+        ],
+    )
+
+    # add explicit model solution
+    ems = flopy.mf6.ModflowEms(
+        sim,
+        pname="ems",
+        filename=f"{prtname}.ems",
+    )
+    sim.register_solution_package(ems, [prt.name])
+
+    return sim
+
+
+# ## Running models
+#
+# Next, build and run the models. The flow model must run before the tracking model so the latter can consume the former's cell budget output file (configured above via flow model interface).
+
+# build mf6 models
+gwfsim = build_gwf_sim(ws, "mf6")
+prtsim = build_prt_sim(ws, "mf6")
+
+# run mf6 models
+for sim in [gwfsim, prtsim]:
+    sim.write_simulation()
+    success, _ = sim.run_simulation()
+    assert success
+
+# Extract model and grid objects from the simulations for use with plots.
+
+# extract model objects
+gwf = gwfsim.get_model(gwfname)
+prt = prtsim.get_model(prtname)
+
+# extract model grid
+mg = gwf.modelgrid
+
+# ## Inspecting results
+#
+# Finally we can load and plot results. First make sure the expected output files were created, then load pathlines, heads, budget, and specific discharge data.
+
+# check mf6 output files exist
+assert (ws / gwf_budget_file).is_file()
+assert (ws / gwf_head_file).is_file()
+assert (ws / prt_track_file).is_file()
+assert (ws / prt_track_csv_file).is_file()
+
+# load pathlines
+pls = pd.read_csv(ws / prt_track_csv_file)
+
+# extract head, budget, and specific discharge results from GWF model
+hds = HeadFile(ws / gwf_head_file).get_data()
+bud = gwf.output.budget()
+spdis = bud.get_data(text="DATA-SPDIS")[0]
+qx, qy, qz = flopy.utils.postprocessing.get_specific_discharge(spdis, gwf)
+
+# Path points/lines can be plotted straightforwardly.
+
+# plot mf6 pathlines in map view
+fig, ax = plt.subplots(nrows=1, ncols=1, figsize=(13, 13))
+ax.set_aspect("equal")
+pmv = flopy.plot.PlotMapView(modelgrid=mg, ax=ax)
+pmv.plot_grid()
+pmv.plot_array(hds[0], alpha=0.1)
+pmv.plot_vector(qx, qy, normalize=True, color="white")
+pathlines = pls.groupby(["imdl", "iprp", "irpt", "trelease"])
+for ipl, ((imdl, iprp, irpt, trelease), pl) in enumerate(pathlines):
+    pl.plot(
+        title="Pathlines",
+        kind="line",
+        x="x",
+        y="y",
+        marker="o",
+        ax=ax,
+        legend=False,
+        color=cm.plasma(ipl / len(pathlines)),
+    )
+
+# Alternatively, with FloPy's built-in pathline plotting utilities:
+
+fig, ax = plt.subplots(nrows=1, ncols=1, figsize=(13, 13))
+ax.set_aspect("equal")
+pmv = flopy.plot.PlotMapView(modelgrid=mg, ax=ax)
+pmv.plot_grid()
+pmv.plot_array(hds[0], alpha=0.1)
+pmv.plot_vector(qx, qy, normalize=True, color="white")
+colors = cm.plasma(np.linspace(0, 1, pls.groupby(["imdl", "iprp", "irpt", "trelease"]).ngroups))
+pmv.plot_pathline(pls, layer="all", colors=colors, linewidth=2)