Add documentation for nutrient split between litter pools

[jacobcook1995]

Description

@hrlai, this adds documentation for the split between of nutrient between pools. Please let me know whether it makes sense (if it doesn't make sense to you it won't make sense to anyone really).

One thing I was wondering is whether it's worth including the equations for how the split is calculated in here? Let me know whether that seems like a good idea or just clutter.

The relevant docs can be viewed here

Fixes #1046

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.56%. Comparing base (745f4b3) to head (fae07db).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1065   +/-   ##
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  Coverage    94.56%   94.56%           
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  Files           79       79           
  Lines         6973     6973           
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  Hits          6594     6594           
  Misses         379      379           

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[hrlai]

Hey @jacobcook1995 yes I think it would benefit (at least someone like me) to have the equations. For now I struggle a bit in parsing the text descriptions.

[jacobcook1995]

@hrlai, ah yes that was a bit of a wall of text. I've broken up the text a bit and added in equations, which should hopefully make it clearer. Let me know what you think!

[hrlai]

Looks good, added very minor edits.

I think it'd be good to add a one-liner interpretation of $r_m$, and I have come up with a one-liner based on the following flow chart. I hope this is the right interpretation.

[jacobcook1995]

Yeah the part in the square brackets is a bit hard to explain.

It basically boils down to the fact that the flow to each pool has already been determined, so the ratio of input chemistries ($\rho$) can only be maintained (consistent with the total input) by changing the chemistry of the input flows ($r_m$ and $r_s$). So the first term in the brackets represents the nutrient input to the metabolic pool and the second term the nutrient input that has to be added to the structural pool to maintain the fixed ratio ($\rho$). The solution to the equation is then the only solution that doesn't involve nutrients being created or destroyed.

[jacobcook1995]

@hrlai , I've tried to explain the above in the docs, let me know if it makes sense or needs further changes

[hrlai]

Ah I see. Thanks, looks good to me now.