calculate sapwood cross-sectional area for grass PFT

[XiulinGao]

Description:

A subroutine, sapwood_area_grass, is added to calculate sapwood cross-sectional area specifically for grass PFT. This is added to fix issue #1254

Collaborators:

Expectation of Answer Changes:

Answers will change is FATES Hydro is turned on and using sapwood allometry mod 2

Checklist

If this is your first time contributing, please read the CONTRIBUTING document.

All checklist items must be checked to enable merging this pull request:

Contributor

• [x] The in-code documentation has been updated with descriptive comments
• [x] The documentation has been assessed to determine if updates are necessary

Integrator

• [x] FATES PASS/FAIL regression tests were run
• [x] Evaluation of test results for answer changes was performed and results provided

Documentation

• Technical Note update:
• User's Guide update:

Test Results:

CTSM (or) E3SM (specify which) test hash-tag:

CTSM (or) E3SM (specify which) baseline hash-tag:

FATES baseline hash-tag:

Test Output:

[glemieux]

Reminder to self:

• [x] Temporarily ESCOMP/CTSM#2878 during final testing

[glemieux]

I conducted some initial regression testing of this by manually modifying the FatesColdHydro test mod to remove the override of fates_allom_smode in the current default parameter file. Both hydro tests pass now using the default param file. Difference are isolated to the relevant hydro related variables and are round off.

[glemieux]

Final regression testing underway. Note that I'm using ESCOMP/ctsm#2882 to test as it captures commits that revert the temporary workaround to #1254 which this PR fixes.

[glemieux]

Regression testing is complete. The FatesColdHydro tests are the only differences with FATES_ROOTUPTAKE variables and FATES_SAPFLOW_SZPF showing the only differences on the first timestep. I confirmed that the hydro tests used the default parameter file as intended. All other expected tests pass b4b.