Revised animal excretion

[TaranRallings]

Description

This is a fairly straightforward change providing a revised metabolic framework. The pre-existing metabolize and excrete methods have been reconfigured into:

• metabolize: for the generating of excreta mass (metabolic wastes)
• excrete: for the removal of nitrogenous wastes into soil pools (urination and analogs)
• respire: for the removal of carbonaceous wastes into the atmosphere (breathing CO2)
• defecate: for the removal of digestive wastes into soil pools

As we are not currently tracking nitrogen or water, the excrete trait and method does nothing but provide structure.

Fixes # (issue)

Type of change

• [x] 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

• [x] Make sure you've run the pre-commit checks: $ pre-commit run -a
• [x] All tests pass: $ poetry run pytest

Further checks

• [x] Code is commented, particularly in hard-to-understand areas
• [x] 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 :white_check_mark:

Project coverage is 94.96%. Comparing base (c60a201) to head (a81376a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #460 +/- ## =========================================== + Coverage 94.75% 94.96% +0.20% =========================================== Files 70 70 Lines 3793 3792 -1 =========================================== + Hits 3594 3601 +7 + Misses 199 191 -8 ```

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