compatability changes for clm bgc

[rgknox]

Description:

These changes are necessary to get FATES compatible with the full BGC call sequence in CLM. Since CLM always has the nitrogen cycle active, with limitation applied by withdolding supplementation, FATES must always present an N litter flux. These changes ensure that in all scenarios (c only, and cnp) we allocated and zero-out the N litter arrays. In the C-only version of fates, we will present the zero fluxes, in the cnp version of fates, these will have real, positive, values.

Synchronized (API change) with CTSM PR 1959

Collaborators:

@adrifoster @billsacks @wwieder @glemieux @ckoven @rosiealice

Expectation of Answer Changes:

We do not expect qualitative changes in FATES runs, wiht the exception of soil related variables, such as SOM, fragmented litter and heterotrophic respiration. These are expected to change in fates runs. These changes are likely to not propogate into FATES-side results, but slight deviations may emerge. non-fates simulations, and fates-sp simulations are not expected to change.

Checklist:

• I have updated the in-code documentation .AND. (the technical note .OR. the wiki) accordingly.
• I have read the CONTRIBUTING document.
• FATES PASS/FAIL regression tests were run (TBD)
• If answers were expected to change, evaluation was performed and provided (see CTSM PR)

Test Results:

TBD

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

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

FATES baseline hash-tag:

Test Output:

[glemieux]

@rgknox and I discussed potentially addressing #697 here as well.