Adding a microbial necromass pool

[jacobcook1995]

Description

This pull request extends the soil carbon model to include a pool for microbial necromass (dead cells, denatured enzymes, etc). As part of this I have changed some of the previous flows between carbon pools.

I've also tried to break the main function used by the soil model integration up a bit (see #344). This is far from complete, but hopefully it's a bit more readable now.

I haven't added any documentation changes to the PR as I'm planning to overhaul the soil model documentation in the near future anyway.

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 79.22078% with 16 lines in your changes missing coverage. Please review.

Project coverage is 94.95%. Comparing base (92de3ee) to head (d8afa00).

Files	Patch %	Lines
...ample_data/generation_scripts/soil_example_data.py	0.00%	10 Missing :warning:
...ple_data/generation_scripts/litter_example_data.py	0.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #477 +/- ## =========================================== - Coverage 94.95% 94.95% -0.01% =========================================== Files 70 70 Lines 3869 3886 +17 =========================================== + Hits 3674 3690 +16 - Misses 195 196 +1 ```

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