Added nitrogen limitation for microbial growth

[jacobcook1995]

Description

This PR adds in nitrogen flows between the full set of soil carbon pools and adds nitrogen as a factor limiting microbial growth. The nitrogen content of the microbial biomass (and enzymes) is not explicitly tracked as microbial biomass is assumed to have fixed stoichiometry (which means it is already tracked implicitly).

This pull request basically just adds a few new variables and new functions to update them, there are not any significant structural changes. Let me know if you notice any areas needing improvement!

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

❌ Patch coverage is 96.15385% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.69%. Comparing base (8873031) to head (a3cbcc7).
⚠️ Report is 2974 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ample_data/generation_scripts/soil_example_data.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #628   +/-   ##
========================================
  Coverage    94.69%   94.69%           
========================================
  Files           73       73           
  Lines         4541     4582   +41     
========================================
+ Hits          4300     4339   +39     
- Misses         241      243    +2     

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[dalonsoa]

Just a couple of suggestions and questions, but looks good!

[dalonsoa]

I guess you have a separate function for this and the chosen inputs as there might be other, more complex stuff going on in the future in this function, right? Otherwise, this functions seems a bit overkilling for a simple division.

[jacobcook1995]

Yep in future, the microbial pool will be separated into (at least) a bacterial pool and a fungal pool. These pools will have different stoichiometry which means the calculation will become more involved, so the function is basically a placeholder for when I will do this.