soil evaporation constraint

[vgro]

This PR makes sure that soil moisture does not fall below residual soil moisture. This was likely the cause of this error occurring during vr_run :

path/virtual_rainforest/virtual_rainforest/models/hydrology/below_ground.py:79: RuntimeWarning: invalid value encountered in sqrt* np.sqrt(effective_saturation)

I didn't add a specific test or warning message.

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

[davidorme]

LGTM - does that change in the test catch the case where the evaporation is constrained not to push SM below RSM? Might be useful to note that?

[codecov-commenter]

Codecov Report

Merging #318 (9ff3727) into develop (10afe8a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #318   +/-   ##
========================================
  Coverage    96.50%   96.50%           
========================================
  Files           48       48           
  Lines         2372     2376    +4     
========================================
+ Hits          2289     2293    +4     
  Misses          83       83           

Files	Coverage Δ
...irtual_rainforest/models/hydrology/above_ground.py	100.00% <100.00%> (ø)
...ual_rainforest/models/hydrology/hydrology_model.py	100.00% <100.00%> (ø)

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[vgro]

LGTM - does that change in the test catch the case where the evaporation is constrained not to push SM below RSM? Might be useful to note that?

It didn't because I made the change in the update function. I added the clip to the function that calculates evaporation so I can test for it and it looks like it works ok.