subset_data point: Fix --create-datm and Longitude TypeErrors

[samsrabin]

Description of changes

My Longitude work introduced a problem that wasn't caught by testing. This PR adds testing and fixes the issue. It also fixes a problem I found where --create-datm was looking for monthly files when it actually should be looking for annual files.

Specific notes

Contributors other than yourself, if any: None

CTSM Issues Fixed:

• Resolves subset_data point fails with TypeError #3258
• Resolves subset_data point --create-datm fails due to bad filename #3260
• Resolves make all-subset error #3197
• Resolves modify subset_data for CRUJRA with a more user-interactive solution #2960

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? No

Testing performed, if any: As of 1244d9e:

• New SystemTests added; failing before fix and passing after.
• Python unit tests pass
• Python system tests
• clm_pymods suite: /glade/derecho/scratch/samrabin/tests_0620-161235de/

[samsrabin]

This PR exposes problems with plumber2_surf_wrapper; those are fixed in PR #3264.

[ekluzek]

@samsrabin and I went over this. There are some things we thought should be done that he has notes for, and I added a few here. This is great work on his part and I'm glad to know it's coming in.

This is a great set of changes that both fix problems and adds testing and better error handling. The error handling would've caught the problems that came up, so good regression testing to keep around. This is another move to having these problem caught earlier, and in an automated fashion, that helps to resolve it when it happens again. Having it in our automated testing also means that it's more likely that just a developer that makes a change will see the problem -- rather than random users. When random users see these problems it's much harder for them to resolve and it potentially affects a bunch of people at once.

This was also a win for doing the parallel work sessions as this came up during the last one on Wednesday.