Use a wrapper type for ThCategory in Opposite cat models

[kris-brown]

Opposite cat models have a piece of data: something which implements ThCategory itself. This is a great use case for wrapper types around models, so this PR shows how to use wrapper types in this context (removing the need to manually check that that the value implements ThCategory).

This also forces us to not rely on GATlab being in the namespace of wherever @wrapper gets expanded. The solution is to use GlobalRef.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.97%. Comparing base (125be0a) to head (106849b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #198      +/-   ##
==========================================
- Coverage   92.18%   91.97%   -0.21%     
==========================================
  Files          39       39              
  Lines        2367     2369       +2     
==========================================
- Hits         2182     2179       -3     
- Misses        185      190       +5     

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