Deprecate C:N:P ratios between plants and litter

[jacobcook1995]

Description

This PR deprecates the variables that go between plants and litter that use the old (one variable per element) style rather than the new elemental mass triplet format.

I've done this for foliage, roots and stems.

However, I'm leaving this PR as a draft as I wasn't sure about the reproductive tissues case. Here, there are several variables marked for deprecation which are generated within allocate_gpp (which isn't a function I feel confident messing around with), these are "plant_reproductive_tissue_turnover", "fallen_non_propagule_c_mass", "plant_rt_turnover_n_mass" and "plant_rt_turnover_p_mass".

I guess the whole flow of fruits etc to litter is going to be replaced with something that occurs via the animals/the seedbank so a lot of this stuff could just be ripped out as it will become redundant soon. But, wanted to check what the plans are before going ahead and doing that

I guess this is something that makes most sense to chat about in person in the new year

Fixes #1131

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

[jacobcook1995]

@sallymatson, I've made the change we discussed yesterday and deleted the reproductive tissue handling entirely from the litter model. It's now ready to review

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.02%. Comparing base (2a48b34) to head (f0d52b6).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1233      +/-   ##
===========================================
- Coverage    95.05%   95.02%   -0.03%     
===========================================
  Files           71       71              
  Lines         7317     7274      -43     
===========================================
- Hits          6955     6912      -43     
  Misses         362      362              

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[sallymatson]

LGTM !! Thanks for making all the deletions, gonna feel so nice in the plants model after this!