Combine data bugfix

[aburrell]

Description

Addresses #142 by removing the assumption that all instruments will have downloaded files that can be queried.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

First, go and delete any data present in these instrument directories.

import datetime as dt
import pysat
import pysatSpaceWeather as pysw

pred = pysat.Instrument(inst_module=pysw.instruments.sw_kp, tag='prediction')
rec = pysat.Instrument(inst_module=pysw.instruments.sw_kp, tag='recent')
start = dt.datetime(2023, 12, 1)
stop = dt.datetime(2024, 2, 1)

kp = pysw.instruments.methods.kp_ap.combine_kp(now, pred, start=start, stop=stop)

forecast = pysat.Instrument(inst_module=pysw.instruments.sw_f107, tag='forecast')
daily = pysat.Instrument(inst_module=pysw.instruments.sw_f107, tag='daily')

f107 = pysw.instruments.methods.f107.combine_f107(daily, forecast, start=start, stop=stop)

With these changes, you will have instruments that have an index and only fill values. Without these changes, you'll get some sort of Exception.

Test Configuration

• Operating system: OS X Big Sur
• Version number: Python 3.10
• Any details about your local setup that are relevant: develop branch of pysat

Checklist:

• Make sure you are merging into the develop (not main) branch
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• Add a note to CHANGELOG.md, summarizing the changes

If this is a release PR, replace the first item of the above checklist with the
release checklist on the pysat wiki:
https://github.com/pysat/pysat/wiki/Checklist-for-Release