feat(netcdf writing)

[spaulins-usgs]

Supports writing data to a MODFLOW-6 readable NetCDF file. Packages with NetCDF write support have a "# flopy netcdf" line in the header of their DFN file. PR includes a set of example projects with expected results and test script that compares flopy's output to the expected results.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.1%. Comparing base (9e87acd) to head (9a03a77).
Report is 67 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2147     +/-   ##
=========================================
- Coverage     73.1%   70.1%   -3.0%     
=========================================
  Files          260     260             
  Lines        59495   58756    -739     
=========================================
- Hits         43494   41216   -2278     
- Misses       16001   17540   +1539     

see 116 files with indirect coverage changes