Update hydraulic failure mortality for places with frozen soils

[JessicaNeedham]

Description:

This PR address issue NGEET#1492 on the NGEET repo.
Hydraulic failure mortality (hmort) occurs when btran is less than a soil moisture threshold. If soils are frozen then btran is zero. To prevent hmort of plants at high latitude during winter there is an if statement that prevents hmort being calculated if any soil layer is frozen. However, in regions with permafrost this essentially turns of hmort entirely, even in summer months when plants may actually be water stressed.

This PR adds additional logic so that we still calculate hmort if soil layers with 90% of the root biomass are not frozen.

Collaborators:

@rosiealice @mvdebolskiy

Expectation of Answer Changes:

This will change hmort rates of plants at high latitudes.

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:

I ran a 5 year test with current code and updates - both with the current parameter file but with hmort scalar set to 0.6 for all PFTs - to make differences easier to see. Not seeing huge differences in the high latitudes.

Here is hmort by PFT for the control:

and with new boreal hmort logic: