activated latent heat flux, introduces shorter indexing

[vgro]

This PR adds latent heat flux to the understorey energy balance. The equation is slightly different,, it has been updated since the first draft.
I also added indices for common layers to avoid these very long lines.

Fixes # 1230

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works
• Relevant documentation reviewed and updated

[jacobcook1995]

LGTM!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.95%. Comparing base (0696c7f) to head (8fccd52).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1231   +/-   ##
========================================
  Coverage    94.95%   94.95%           
========================================
  Files           71       71           
  Lines         7288     7293    +5     
========================================
+ Hits          6920     6925    +5     
  Misses         368      368           

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