Animal Module constants rework

[TaranRallings]

Description

This is the rework of constants handling in the animal module. Everything seems to be working but do let me know if I've misunderstood something about implementing constants in this way.

Fixes # (issue)

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
• [x 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

[davidorme]

LGTM. You may need to add another constants class to the model if you end up using core constants (and this may just become part of the BaseModel signature) but see also #342 - this is still up in the air, so just to be aware of.