Carbon allocation bug fix

[mpaiao]

Revised the carbon allocation routine to ensure allocation to fine roots and leaves are truly proportional to demand.

Description:

1. Bug fix in DailyPRTAllometricCarbon (parteh/PRTAllometricCarbonMod.F90). When allocating to different tissues, the code was subtracting allocation of tissues before calculating the amount for the next tissue, potentially under-allocating carbon to fine roots.
2. Minor code updates to simplify allocation calculations. It now uses a single
   allocation factor based on availability and total need, which is applied to all tissues.
3. Replaced a few if statements with select case, which simplifies adding other hypotheses
   in the future (and it's safer for selecting cases).

This pull request addresses the bug discussed in #784 and supersedes pull request #800.
Additional minor changes in former PR #800 will be added as subsequent pull requests.

Collaborators:

@rgknox @glemieux @jkshuman

Expectation of Answer Changes:

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
• 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 against baseline fates-sci.1.57.2_api.23.0.0-ctsm5.1.dev098 is complete. There are no unexpected failures. The satellite phenology and static stand structure test mods all pass B4B which is expected. DIFFs appear to be as expected with initial and daily small differences in carbon balance and related variables. Interestingly the PRT2 testmod passes B4B, but I'm not sure why this would be so. @rgknox do you have thoughts here?

Results can be found on Cheyenne at:
/glade/u/home/glemieux/scratch/ctsm-tests/tests_pr863-sci_1_57_2

[rgknox]

That makes sense, the PRT2 has a completely different allocation module, only the carbon-only code was changed. I believe the PRT2 module does not have the bug that @mpaiao is fixing, so that should be fine