Fix units soil moisture equation for ch

[LINAMARIAOSORIO]

Checklist

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Added tests for changed code.
• Updated documentation for changed code.

Description

This PR fixes the calculation of the soil moisture condition coefficient C_h in SurfaceRunoff.get_coef_soil_moist_conditions().

The previous implementation mixed a dimensionless soil moisture quantity in the numerator with the saturation storage in millimeters in the denominator.

Related Issue

• Resolve C_h mixes dimensionless soil moisture with TUsat in mm #318 ;

Motivation and context

• TBF

How has this been tested

• TBF

Screenshots

• N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.48%. Comparing base (92d45d6) to head (16922cd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #321      +/-   ##
==========================================
- Coverage   92.48%   92.48%   -0.01%     
==========================================
  Files          60       60              
  Lines        3911     3910       -1     
  Branches      487      487              
==========================================
- Hits         3617     3616       -1     
  Misses        238      238              
  Partials       56       56              

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