Added step to catch infinite stochiometries

[jacobcook1995]

Description

I've added a new function to the litter model code that should catch any case where input organic matter stoichiometries or chemical proportions (in the case of lignin) go negative. This new function raises an error in this case as we have decided to treat this case as an error.

Fixes #732

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 94.74%. Comparing base (ccc87e9) to head (3a54505).
Report is 705 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #733      +/-   ##
===========================================
+ Coverage    94.73%   94.74%   +0.01%     
===========================================
  Files           73       73              
  Lines         4974     4984      +10     
===========================================
+ Hits          4712     4722      +10     
  Misses         262      262              

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[TaranRallings]

LGTM!