modify subset_data for CRUJRA (currently hardcoded for GSWP3 and rather brittle).

[ekluzek]

subset_data is hardwired to work with datm_gswp3.
The default datm dataset is changing for clm6 to CRUJRA2024.

I added this comment to subset_data.py in #2956:

    # TODO issue #1895: allow datm_crujra, which also affects
    #      tools/site_and_regional/default_data_1850.cfg
    #      tools/site_and_regional/default_data_2000.cfg
    #      python/ctsm/test/testinputs/default_data.cfg

and posted this comment.

[wwieder]

I'll note that I have an equally hard and brittle application of subset data that works for CRUJRA (currently unavailable on Derecho).

• In the short term, we can bring this option in (having something that's functional with CRUJRA seems critical).
• Alternatively, maybe it's not too difficult to make less brittle python code that can handle both options? (not sure how much we can support multiple datm products in our subsetting tools)?