Fix to C starvation by canopy layer history variables

[JessicaNeedham]

Description:

This PR fixes issue #970 by updating the history variables related to carbon starvation mortality by canopy layer:
FATES_M3_MORTALITY_CANOPY_SZPF, FATES_M3_MORTALITY_CANOPY_SZ, FATES_M3_MORTALITY_USTORY_SZPF, and FATES_M3_MORTALITY_USTORY_SZ.

Collaborators:

Expectation of Answer Changes:

These values would previously be zero but should now reflect the number density of cohorts dying by carbon starvation mortality in each canopy layer. Only tested with E3SM c63cce2 and FATES e663a6e. Test results below are from my branch here:
https://github.com/JessicaNeedham/fates/tree/JFN-bagw-vars-respmod-v2

Canopy layer outputs are compared with FATES_MORTALITY_CSTARV_SZPF.

Checklist:

• My change requires a change to the documentation.
• 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
• [ X] If answers were expected to change, evaluation was performed and provided

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]

Regression testing on cheyenne is complete. I tested this and #926 at the same time by merging them together. All expected tests pass b4b against fates-sci.1.63.0_api.25.1.0-ctsm5.1.dev116 with the exception of field list differences.

Folder location: /glade/u/home/glemieux/scratch/ctsm-tests/tests_pr971_926-dev116