replaced water_to_air_mass_ratio in abiotic constants with molecular_…

[vgro]

…weight_ratio_water_to_dry_air from core constants

This PR replaces a duplicated constant.

Fixes #849

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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.76%. Comparing base (bd4c1f6) to head (dce08d3).
⚠️ Report is 2106 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #850      +/-   ##
===========================================
- Coverage    94.76%   94.76%   -0.01%     
===========================================
  Files           75       75              
  Lines         5563     5561       -2     
===========================================
- Hits          5272     5270       -2     
  Misses         291      291              

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[sallymatson]

lgtm!