Add flow of excrement decay into the soil model

[jacobcook1995]

Description

This PR changes the soil model so that it now actually uses the excrement and carcass flows. For this exchange I've now started using the CNP triplets so that I only need to use 2 variables for the exchanges rather than 6.

Fixes #1177

@sallymatson and @TaranRallings, I've added both of you as reviewers because I want to check that I have used the CNP triplets correctly and that I haven't messed up anything in the animal model. No rush at all with reviewing this, I just wanted to get the PR made before I went on annual leave

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 95.03%. Comparing base (0f4f9ed) to head (2633bf6).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1224   +/-   ##
========================================
  Coverage    95.03%   95.03%           
========================================
  Files           71       71           
  Lines         7278     7278           
========================================
  Hits          6917     6917           
  Misses         361      361           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[TaranRallings]

Straightforward enough, LGTM!

[sallymatson]

LGTM ! Left one minor comment but not blocking by any means