Update single point docs (and improve subset_data etc. erroring)

[samsrabin]

Description of changes

• Updates single-point documentation to reflect the current workflow
• Adds error messages in subset_data and related scripts to give descriptive error messages when the user tries something that isn't supported and/or is buggy

Corresponding ctsm-docs PR: ESCOMP/ctsm-docs#8

Remaining work:

• Have subset_data add the ./xmlchange DATM years commands to shell_commands (as mentioned in Fix bug or update documentation for "How do I create a single-point run with the NUOPC (default) coupler?" #2890)

Specific notes

Contributors other than yourself, if any: Katie Rocci, @adrifoster

CTSM Issues Fixed:

• Contributes to Documentation for single point simulations, especially NEON #1494
• Resolves Documentation need for generic single point with nuopc #1565
• Deprecates How do I create a single-point run with the NUOPC (default) coupler? #1833, which should be deleted (or noted that it's deprecated)
• Contributes to Documentation of new features needed #2113
• Resolves Convert "How do I create a single-point run with the NUOPC (default) coupler?" to real docs #2892
• Resolves Throw errors in subset_data for known buggy setups #2999

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

Testing performed, if any:

As of 7c1d2dc:

• Python unit tests
• Python system tests

As of 685e08e (but nothing changed since then is tested in this suite):

• clm_pymods test suite

As of 79e202d:

• Python unit tests
• Python system tests
• Documentation build

[ekluzek]

A short skim of this and I'm liking what I see. Thanks for starting work on this. The simple note you added about single point is a great starting point to have in place.

[samsrabin]

@ekluzek Sorry for the many pings on this as it went ready/draft/ready, but it's really ready now. Could you please review today so I can merge it before b4b-dev is locked tomorrow?

[ekluzek]

The code is great here. I have a long discussion about what to do with the modify_fsurdat unit tests that had to change. I recommend changing it to delete that code, but I'll leave it up to you.

I have several suggestions on the documentation. The main of which I already brought up and we agreed to leave the PTS_MODE documentation, but don't highlight it.

I'm marking this as approve at this point so you can bring it in without tracking me down again for reapproval.

[samsrabin]

• @slevis-lmwg will add documentation to this about how to use GSWP3 instead of CRU-JRA in subset_data (see modify subset_data for CRUJRA with a more user-interactive solution #2960)