Don't zero out patch%fuel%frac_burnt() before we use it

[adrifoster]

Fixes #1301 by checking inside the TransLitterNewPatch for the type of disturbance instead of zero-ing outside of the subroutine.

Description:

Removed the lines in spawn_patches that zero-d the frac_burnt array too soon.
Added checks in TransLitterNewPatch to check for fire disturbance and a fire on the patch:
If no fire, new local variable frac_burnt set to 0.0, otherwise use patch%fuel%frac_burnt

Collaborators:

@glemieux

Expectation of Answer Changes:

Yes - should change answers because before we weren't burning any fuel

Checklist

Contributor

• The in-code documentation has been updated with descriptive comments
• The documentation has been assessed to determine if updates are necessary

Integrator

• FATES PASS/FAIL regression tests were run
• Evaluation of test results for answer changes was performed and results provided

Test Results:

CTSM (or) E3SM (specify which) test hash-tag: ctsm5.3.012

CTSM (or) E3SM (specify which) baseline hash-tag: ctsm5.3.012

FATES baseline hash-tag: sci.1.80.4_api.37.0.0

Test Output:

Running tests now

[glemieux]

I like moving the check into TransLitterNewPatch. That said, do we need to make sure that currentPatch%fuel%frac_burnt is set to zero at the end of spawn_patches or is that already handled elsewhere?

[adrifoster]

I like moving the check into TransLitterNewPatch. That said, do we need to make sure that currentPatch%fuel%frac_burnt is set to zero at the end of spawn_patches or is that already handled elsewhere?

I added it to the top of the fire_model subroutine, where we zero other fire-related things. I don't know if this is the right place for it or not...