JP-3721: Simplify ModelContainer

[emolter]

Resolves JP-3721

Closes #8738

This PR enables the JWST ModelContainer to no longer inherit from DataModel and to no longer require its own save method, as part of an effort to make ModelContainer's usage narrower and easier to understand.

Tasks

• update or add relevant 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)
  • run jwst regression tests with this branch installed ("git+https://github.com/<fork>/stdatamodels@<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 66.45%. Comparing base (1996b18) to head (7d5798e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #330   +/-   ##
=======================================
  Coverage   66.44%   66.45%           
=======================================
  Files         102      102           
  Lines        5439     5440    +1     
=======================================
+ Hits         3614     3615    +1     
  Misses       1825     1825           

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