Add FATES interstitial bareground handling namelist option

[glemieux]

Description of changes

This pull request adds a namelist option to change how the FATES "interstitial" bareground is handled. Interstitial bareground in this context means the fraction of a given FATES patch area that is not covered by vegetation. Currently, the interstitial bareground across all FATES patches on a given site is accounted for in the HLM bareground patch (pfti). If the new namelist option use_fates_interstitial_bareground is .false. then the bareground patch weight (wt_ed) is simply zero.

Specific notes

This pull request should be coordinated with NGEET/fates#1577.

Contributors other than yourself, if any:

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CTSM issues resolved or otherwise addressed, if any:

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If answers are expected to change, describe (delete this line otherwise):

Any user interface changes (namelist or namelist defaults changes)?

Testing planned or performed, if any:

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Requirements before merge:

• I have followed the CTSM contribution guidelines.

• The code in this PR branch builds with no errors.
• The code in this PR branch runs with no errors. Briefly describe tested configuration(s):
• This either (a) does not change answers, (b) it only changes answers at roundoff level, or (c) I have performed a scientific evaluation of the answer changes. Which?:

• I have reviewed relevant parts of the CLM documentation Tech Note or User's Guide to determine if anything needs to be changed or added. If it does, describe:
• This PR either (a) does not create a need to update the documentation or (b) includes required documentation updates (see guidelines for contributing documentation). Which?: