Add phosphorus to litter model

[jacobcook1995]

Description

This pull requests adds tracking of litter phosphorus concentrations. This follows an identical structure to the nitrogen case. I did consider trying to make a generic functions rather than duplicating functions, but it didn't feel worth the effort as we have no current plans to add further nutrients (e.g. Potassium).

As there weren't any structural changes I didn't split this up into separate PRs, hopefully it is a manageable size as a single PR.

Fixes #512

Type of change

• [x] 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

• [x] Make sure you've run the pre-commit checks: $ pre-commit run -a
• [x] All tests pass: $ poetry run pytest

Further checks

• [x] Code is commented, particularly in hard-to-understand areas
• [x] Tests added that prove fix is effective or that feature works
• [ ] Relevant documentation reviewed and updated

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 94.95%. Comparing base (ca04b51) to head (76f76f9).

Files with missing lines	Patch %	Lines
...ple_data/generation_scripts/litter_example_data.py	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #539 +/- ## =========================================== - Coverage 94.98% 94.95% -0.03% =========================================== Files 74 74 Lines 4145 4186 +41 =========================================== + Hits 3937 3975 +38 - Misses 208 211 +3 ```

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