Changed litter model to use exact solution for the litter pool size

[jacobcook1995]

Description

This PR changes the litter model to use an exact analytic solution for the pool size with time. The new model approach is in this part of the docs.

While making this change I realised that the way I was averaging nutrient ratios was incorrect. I decided it made most sense to fix that at the same time, but that means that this PR has ballooned in size

@vgro, this fixes the issue of the negative litter pools crashing ve_run when the abiotic model is included. Unfortunately, it still doesn't run, but it's now a soil model error (linked to #536) so that's progress

@hrlai, I've add you as a reviewer but don't worry about looking at the code, if you could sense check the documentation + check that it fits with the models that you are fitting for the parameterisation, that would be great!

Fixes #979, Fixes #991, Fixes #969

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 94.90%. Comparing base (266dbe6) to head (52474f4).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1001      +/-   ##
===========================================
+ Coverage    94.82%   94.90%   +0.08%     
===========================================
  Files           79       80       +1     
  Lines         6511     6616     +105     
===========================================
+ Hits          6174     6279     +105     
  Misses         337      337              

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[vgro]

Looks sensible to me, I cannot comment much on the science

[hrlai]

Looks good, I've read the theory document only and it makes sense.

[jacobcook1995]

Great, thanks both of you! I've made the suggested docs changes and will go ahead and merge now