generate_gdds: Work on arguments

[samsrabin]

Description of changes

Various additions to arguments of generate_gdds.py. Adds capability but shouldn't change default answers.

• Adds argument testing.
• Renames --unlimited-season-length to --max-season-length-from-hdates-file to more accurately reflect what it does.
• Adds --paramfile argument, which is required when the above is not provided, because if not getting max season length from harvest dates file it'll come from a paramfile. (Mutually exclusive with the above.)
• Refactors and tests code for getting max season length.
• Fail if provided sowing and harvest date files don't match the CLM grid.

Specific notes

Contributors other than yourself, if any: None

CTSM Issues Fixed (include github issue #):

• Contributes to Rethinking (variables in) h2 files for GDD-generating workflow #3319

Are answers expected to change (and if so in what way)? No

Any User Interface Changes (namelist or namelist defaults changes)? No

Does this create a need to change or add documentation? Did you do so? Yes; no. Will come in once #3319 is done.

Testing performed, if any:
Tested as part of my crop reparameterization work.

• Python unit testing
• Python system testing
• clm_pymods test suite
• rxcropmaturity test suite

[slevis-lmwg]

@samsrabin gave me a detailed tour of the PR. We discussed a couple of questions that I had along the way. I did not see showstoppers, so I'm approving.