Clean up some parts of datamodels._core

[WilliamJamieson]

Resolves RCAL-612

Closes #247

This PR cleans up parts of the datamodels._core module and adds the ability to properly initialize an "empty" data model, in so far as that if an empty model is created, one can now successfully add information to it. However, the empty model will not validate until it is fully flushed out, so the recommendation is still to use the appropriate maker_util.

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/294/. The failure appears to be the intermittent ASDF file open failure we sometimes get when a the network gets a bit slow.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
tests/conftest.py	100.00% <100.00%> (ø)
tests/test_models.py	100.00% <100.00%> (ø)
tests/test_stnode.py	100.00% <100.00%> (ø)
src/roman_datamodels/datamodels/_core.py	93.04% <97.61%> (+1.46%)	⬆️

... and 2 files with indirect coverage changes

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[braingram]

Thanks for addressing the comment. LGTM.