Snagging the config system switch over in science models

[davidorme]

Description

This PR catches a number of things that snuck through some of the other model conversion PRs. Notably there was a lot of usage of old style config stuff in the litter model that I missed. A few in the plants model.

I also added some new constants fixtures using the new system to reduce the amount of importing in tests and to avoid repeatedly creating constants classes in testing.

Fixes #1123

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 94.69%. Comparing base (834a8df) to head (8310cf6).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1124   +/-   ##
========================================
  Coverage    94.69%   94.69%           
========================================
  Files           90       90           
  Lines         7872     7872           
========================================
  Hits          7454     7454           
  Misses         418      418           

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