Deprecate extra_fits

[emolter]

Closes #295

Resolves JP-3886

This PR deprecates direct reading and setting of the extra_fits and _extra_fits attributes of datamodels.

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 78.15%. Comparing base (4e83608) to head (cb57475).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #402      +/-   ##
==========================================
- Coverage   78.16%   78.15%   -0.02%     
==========================================
  Files         115      115              
  Lines        5144     5158      +14     
==========================================
+ Hits         4021     4031      +10     
- Misses       1123     1127       +4     

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[emolter]

@braingram I have to admit I've never had to deprecate access to an attribute before, nor deprecate something that is currently used in so many places. Leaving this as draft for now - is this anywhere near what you would have envisioned?

[braingram]

@braingram I have to admit I've never had to deprecate access to an attribute before, nor deprecate something that is currently used in so many places. Leaving this as draft for now - is this anywhere near what you would have envisioned?

Thanks. Yeah I think this is on the right track. Since we're awaiting feedback I'll defer a detailed look until we've heard back.