Move stnode into a sub-package

[WilliamJamieson]

This PR moves stnode into a sub-package and then splits module apart into separate modules each handling one part of the functionality of stnode. In addition to the split, I have adjusted the names of some of the private functions and added further documentation of stnode. Hopefully, the split of responsibilities combined with the additional inline code documentation will make stnode easier to maintain.

In addition, to the split/document, I also simplified a few parts of the code:

1. I removed the metaclass based registry of node classes, and replaced it with __init_subclass__ registry. The metaclasses are more than we need for this application.
2. I made all STNode classes dynamically generated (solved scalar/list node corner case).
3. As part of the dynamic changes, I changed the code to using a mixin based system. That is for nodes that we need to define additional code on, instead of making a direct subclass, instead I changed it to a mixin which is automatically detected and mixed into the class dynamicall.

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/241/

[codecov]

Codecov Report

Patch coverage: 91.98% and project coverage change: +0.22 🎉

Comparison is base (b7c92c3) 94.25% compared to head (fabcc6f) 94.48%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #213      +/-   ##
==========================================
+ Coverage   94.25%   94.48%   +0.22%     
==========================================
  Files          18       25       +7     
  Lines        1742     1778      +36     
==========================================
+ Hits         1642     1680      +38     
+ Misses        100       98       -2     

Impacted Files	Coverage Δ
src/roman_datamodels/stnode/_tagged.py	87.30% <87.30%> (ø)
src/roman_datamodels/stnode/_node.py	87.97% <87.97%> (ø)
src/roman_datamodels/stnode/_factories.py	96.07% <96.07%> (ø)
src/roman_datamodels/stnode/__init__.py	100.00% <100.00%> (ø)
src/roman_datamodels/stnode/_converters.py	100.00% <100.00%> (ø)
src/roman_datamodels/stnode/_mixins.py	100.00% <100.00%> (ø)
src/roman_datamodels/stnode/_registry.py	100.00% <100.00%> (ø)
src/roman_datamodels/stnode/_stnode.py	100.00% <100.00%> (ø)

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[perrygreenfield]

Other than a minor docstring error, it looks good. I had to refresh my brain by looking at the original code (even though I wrote much of it, but not the dynamic aspects) and that took some time.

[perrygreenfield]

This doesn't match the function signature

[perrygreenfield]

LGTM

[perrygreenfield]

LGTM