alter hydraulic failure mortality logic

[rosiealice]

This is a minor change to how hydraulic failure mortality is calculated.
Previously, below the threshold, mortality occured at the rate dicated by the mort_scalar_hydrfailure
This lead to behaviour with very abrupt thresholds and no sensitivity to altered soil moisture status in long vs short droughts.
This logic was nver important before because the default hf_sm_threshold parameters were very close to zero. Making them larger brings this mechanism into play.
The function, as all the other mortality functions, is a bit arbitrary and scales mortality to be equal to the mort_scalar_hydrfailure when btran is zero. Below the hf_sm_threshold mortality begins to ramp up as a function of the difference between btran and the threshold.

Description:

Collaborators:

Expectation of Answer Changes:

This will changes answers, in dry places.
The differences introduced by this PR are illustrated here:
https://ns9560k.web.sigma2.no/datalake/diagnostics/noresm/rosief/i2000.f45_f45_mg37.fates-nocomp.beta01.v22i/compare/i2000.f45_f45_mg37.fates-nocomp.beta01.v22h/

Here for example is the change in LAI, which as anticipated is mostly in semi arid regions. The magnitude of the effect depends on the settings for the relevant parameters (these are largely unknown, and part of the purpose of FATES is to introduce and expose these types of mortality mechanisms to validation and calibration).

Checklist:

• [ x] My change requires a change to the documentation. This would be true if it were being wrapped back into the main FATES repo
• I have updated the in-code documentation .AND. (the technical note .OR. the wiki) accordingly.
• [ x] 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:

[JessicaNeedham]

Thanks @rosiealice this looks like a good and necessary change. Noting that per discussion, this change is similar to PR NGEET#1427 on the FATES NGEET repo but should be brought in here first since it might be a while before that PR is integrated. One minor suggestion to prevent negative hmort.