Adding a litter model

[jacobcook1995]

Description

This pull request adds a litter model to the Virtual Rainforest. The purpose of this model is to handle the decomposition of complex biomass lying on or just under the forest floor.

I made this a separate model from the soil model for a couple of reasons:

1. There are plausible use cases (mainly animal focused) where you would want to track the litter (as a resource for consumption) but not the full soil nutrient cycles.
2. There's an alternative microbially explicit approach to modelling litter decay that I would like to try (if I get the time). I feel like this would be easier to implement with the litter model separated from the soil model.

At the moment, the model only has two pools "above_ground_metabolic" and "above_ground_structural" which represent the fast and slow decomposing above ground litter, respectively. In future, below ground pools and wood pools will also be added. The model will also get quite a lot more complex when we add leaf chemistry in (lignin, N and P).

I originally planned to add links from this model to the soil model in, but I thought that pull request was already getting large enough, so decided to save that for a later pull request.

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

[alexdewar]

I've got some minor queries/suggestions, but otherwise LGTM!

[davidorme]

Not a lot to add to @alexdewar. LGTM

[codecov-commenter]

Codecov Report

Merging #268 (81c18f8) into develop (6e4c413) will increase coverage by 0.26%.
Report is 11 commits behind head on develop.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #268      +/-   ##
===========================================
+ Coverage    95.27%   95.54%   +0.26%     
===========================================
  Files           43       47       +4     
  Lines         1778     1885     +107     
===========================================
+ Hits          1694     1801     +107     
  Misses          84       84              

Files Changed	Coverage Δ
virtual_rainforest/models/soil/carbon.py	100.00% <ø> (ø)
virtual_rainforest/models/soil/soil_model.py	100.00% <ø> (ø)
virtual_rainforest/models/hydrology/constants.py	100.00% <100.00%> (ø)
...ual_rainforest/models/hydrology/hydrology_model.py	100.00% <100.00%> (ø)
virtual_rainforest/models/litter/__init__.py	100.00% <100.00%> (ø)
virtual_rainforest/models/litter/constants.py	100.00% <100.00%> (ø)
virtual_rainforest/models/litter/litter_model.py	100.00% <100.00%> (ø)
virtual_rainforest/models/litter/litter_pools.py	100.00% <100.00%> (ø)

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[TaranRallings]

LGTM!