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
• The documentation has been assessed to determine if updates are necessary

Integrator

• FATES PASS/FAIL regression tests were run
• 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: