Adding inorganic phosphorus pools to the soil model

[jacobcook1995]

Description

This pull request adds three inorganic phosphorus pools to the soil model. One of these forms of phosphorus (labile phosphorus) can be used by microbes so this pull requests alters that part of the model, as now there are two sources of phosphorus that microbes can grow from. The other two pools don't do all that much and basically just exist as stores of inorganic phosphorus in the soil, as such transfers involving these pools are just governed by slow linear kinetics.

Based on @dalonsoa comments on my last PR I've tried to replace returned dictionaries with dataclasses to make the code clearer.

Obviously this is a pretty science heavy PR but feedback on code style etc would be useful. I need to make further tweaks to this section of code in the immediate future, so feel free to suggest longer term improvements if they occur.

Fixes #663

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

Attention: Patch coverage is 97.85714% with 3 lines in your changes missing coverage. Please review.

Project coverage is 94.63%. Comparing base (017c70f) to head (4dc09cb).

Files with missing lines	Patch %	Lines
...ample_data/generation_scripts/soil_example_data.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #672      +/-   ##
===========================================
+ Coverage    94.56%   94.63%   +0.07%     
===========================================
  Files           73       73              
  Lines         4634     4757     +123     
===========================================
+ Hits          4382     4502     +120     
- Misses         252      255       +3     

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[sallymatson]

It all looks good to me, and the changes to dataclasses make it clearer. To be honest I don't know the code architecture well enough yet to feel confident in giving approval just bc I can't really think through the implications, so I'll let the other's do that :)

[dalonsoa]

I'm at a conference this week and cannot review this in detail. If you can wait until next week, great! Otherwise, just move on if there're enough positive reviews.

[alexdewar]

I can't comment on the science, but the code changes seem reasonable and clear.