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/.feature.rst``: new feature - ``changes/.bugfix.rst``: fixes an issue - ``changes/.doc.rst``: documentation change - ``changes/.removal.rst``: deprecation or removal of public API - ``changes/.misc.rst``: infrastructure or miscellaneous change

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.