[python] Append-mode label/ID-mapping logic

[johnkerl]

Issue and/or context:

Tracking issue: #1278

Changes:

• Create the classes ExperimentAmbientLabelMapping, AxisAmbientLabelMapping, ExperimentIDMapping, and AxisIDMapping
• None of these will be exposed to the user; they are internal-only.

Notes for Reviewer:

• This is a split-out PR -- created for mercifulness to reviewers -- as a line-count reduction from raw material on [WIP] [python] Append-mode sketching #1498. See also [python] Append-mode pre-check logic #1554.
• See also [python] Append-mode pre-check logic #1554 which is another, orthogonal component.
• Actual use by the ingestor logic will be on a subsequent PR.

[codecov-commenter]

Codecov Report

Patch coverage: 84.81% and project coverage change: +26.15% 🎉

Comparison is base (e26eeac) 64.82% compared to head (d682f92) 90.97%.

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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1556       +/-   ##
===========================================
+ Coverage   64.82%   90.97%   +26.15%     
===========================================
  Files         105       34       -71     
  Lines        8788     2860     -5928     
===========================================
- Hits         5697     2602     -3095     
+ Misses       3091      258     -2833     

Flag	Coverage Δ
python	90.97% <84.81%> (+0.08%)	⬆️
r	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...thon/src/tiledbsoma/io/registration/id_mappings.py	78.12% <78.12%> (ø)
...ledbsoma/io/registration/ambient_label_mappings.py	85.89% <85.89%> (ø)
.../python/src/tiledbsoma/io/registration/__init__.py	100.00% <100.00%> (ø)

... and 86 files with indirect coverage changes

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[thetorpedodog]

This was actually quite a bit easier to get through than I expected! Overall very well written and documented. I have a few minor suggestions but otherwise no concerns from me.

[johnkerl]

@thetorpedodog ready for round two! :)

[johnkerl]

@thetorpedodog ping 🙏

[thetorpedodog]

A few notes I think I might have missed before. Sorry for losing track of this!

[johnkerl]

@thetorpedodog ready for round three!

[thetorpedodog]

A couple small, optional suggestions, but this looks good to me overall. May still want to have a domain expert check it over but it looks like it does everything it should and stylistically it looks great.

[johnkerl]

@mojaveazure @aaronwolen thoughts? Again this is just a scaffolding PR, ingest-per-se coming as a follow-up ...

[johnkerl]

Happy to take further belated feedback on other PRs on #1278