890 mycorrhizal fungi should be obligate rather than able to survive via saprotrophy

[jacobcook1995]

Description

This PR changes the function that calculate mycorrhizal carbon acquisition and nutrient uptake so that only plant supplied carbon is used for growth. Nitrogen and phosphorus can still be obtained from soil organic matter, but mycorrhiza will now try and obtain inorganic nutrients first.

I also altered the test parameterisation of the arbuscular mycorrhizal fungi, so that they don't produce enzymes. Down the line I need to considering preventing them from producing enzymes at all.

Fixes #890

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 93.97%. Comparing base (25e34d7) to head (2e02f97).
⚠️ Report is 1950 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #893      +/-   ##
===========================================
- Coverage    93.97%   93.97%   -0.01%     
===========================================
  Files           75       75              
  Lines         5813     5811       -2     
===========================================
- Hits          5463     5461       -2     
  Misses         350      350              

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[vgro]

LGTM