Fixes for reference file unit removal

[WilliamJamieson]

Resolves RCAL-nnnn

Closes #

This PR makes roman_datamodels compatible with the reference file unit removals in spacetelescope/rad#490.

Tasks

• Update or add relevant roman_datamodels tests.
• Update relevant docstrings and / or docs/ page.
• Does this PR change any API used downstream? (If not, label with no-changelog-entry-needed.)
  • Write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types).
  • Start a romancal regression test (https://github.com/spacetelescope/RegressionTests/actions/workflows/romancal.yml) with this branch installed ("git+https://github.com/<fork>/rad@<branch>").

News fragment change types:

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: fixes an issue
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.misc.rst: infrastructure or miscellaneous change

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.74%. Comparing base (087a60d) to head (388cc73).
Report is 57 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #408      +/-   ##
==========================================
+ Coverage   97.56%   97.74%   +0.18%     
==========================================
  Files          30       37       +7     
  Lines        2788     3416     +628     
==========================================
+ Hits         2720     3339     +619     
- Misses         68       77       +9     

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