Description of changes

In the existing code, when using the default CLM crop calendars, gddmaturity (maturity requirement; the number of growing degree-days it takes for a crop to mature and be harvested) can vary within some bounds based on climate over the past 20 years. Specifically, CLM looks at some annual "warm period" which differs between the northern and southern hemispheres. This variation allows "farmers" to adapt the "cultivars" (varieties) they grow based on recent climate.

When prescribing maturity requirement, though—with input file stream_fldFileName_cultivar_gdds—it's currently not allowed to vary over time. Instead, it's locked in to what's provided in the input file: the mean GDD accumulation in the GGCMI growing period over the 1980-2009 growing seasons.

We want this variation to allow "farmers" to adapt the "varieties" they grow based on whether the recent climate has been warmer or cooler than that baseline 1980-2009 period.

This PR represents the first step toward adding that ability. I'm breaking this work up into separate PRs to keep them manageable:

1. (This PR) Define "recent climate" based on the same "warm period" currently used by default CLM.
   1.1. Add more namelist options and checks.
2. Define "recent climate" based on the GGCMI growing seasons.
3. Restrict adapted gddmaturity values to a realistic range.

Specific notes

Contributors other than yourself, if any: None

CTSM Issues Fixed (include github issue #):

• Contributes to "Easy wins" for crop calendars #1928

Are answers expected to change (and if so in what way)? Not unless enabling this new, experimental feature.

Any User Interface Changes (namelist or namelist defaults changes)?

• Adds input file stream_fldFileName_gdd20_baseline (default ''). Pointing this at a file automatically enables adaptation of prescribed maturity requirements; there's no separate boolean namelist variable to worry about. Providing this file but not prescribed maturity requirements will cause an error in the build namelist step.

Does this create a need to change or add documentation? Did you do so? Documentation will need to be updated when all the PRs in my plan are merged and this feature is considered ready for use. But at this point, things are preliminary enough that I'd have to rewrite whatever docs I added in this PR. So no: I'm adding no documentation here.

Testing performed, if any:

• Ensure clm_short passes bit-for-bit
• Ensure a transient run with stream_fldFileName_gdd20_baseline provided works as we expect (gddmaturity changes over time)
• Ensure aux_clm passes bit-for-bit