Make model subscription follow the same reasoning as node subscription

[WilliamJamieson]

Datamodels are currently not subscribable and setting through subscription acts differently than it does for nodes. This is surprising behavior. This PR brings subscription for datamodels in line with the expectations for node objects.

Checklist

• Added entry in CHANGES.rst under the corresponding subsection
• updated relevant tests
• updated relevant documentation
• Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/Roman-Developers-Pull-Requests/610/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.58%. Comparing base (087a60d) to head (6aab3f0).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #322      +/-   ##
==========================================
+ Coverage   97.56%   97.58%   +0.02%     
==========================================
  Files          30       31       +1     
  Lines        2788     2814      +26     
==========================================
+ Hits         2720     2746      +26     
  Misses         68       68              

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[PaulHuwe]

LGTM

[schlafly]

@WilliamJamieson is out. Can one of you run the regtests and then merge? Thanks!

[PaulHuwe]

I am on it. regtests passed and linked. Will merge after unit tests pass.