subset_data shell commands should explicitly set DATM_MODE

[wwieder]

Subset data will nicely subset DATM from CRUJRA2024b for point runs and create usermods, but the following logic seems to be missing:

If using --create-datm and --create-user-mods flags the shell commands should also include:
./xmlchange DATM_MODE=CLMCRUJRA2024b

[wwieder]

I also noticed that subset data defaults to the 16pft surface dataset now. I'm assuming that's what we want, but was surprised it was not the 78 pft dataset by default?

[ekluzek]

The DATM mode comes from the long compset name. I don't think we should override that in CTSM user-mods, unless there is a really, really important reason to do so. This is part of making sure that our compsets are setup in the way that we want them to be.