Stop negative growth rates from resulting in negative enzyme productions

[jacobcook1995]

Description

This PR changes the calculation of enzyme production so that when microbial growth rates are negative (which can happen if mycorrhiza can't meet nutrient demands) the enzyme production rates don't go negative

Fixes #892

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 93.96%. Comparing base (21a5636) to head (917b82d).
⚠️ Report is 1857 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #920   +/-   ##
========================================
  Coverage    93.96%   93.96%           
========================================
  Files           77       77           
  Lines         5862     5864    +2     
========================================
+ Hits          5508     5510    +2     
  Misses         354      354           

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[vgro]

LGTM