Refactor VaryingCelestialTransform models #876

Job	Run time
sdist_verify	15s
allowed-fail-tests / Load tox environments	4s
docs / Load tox environments	8s
tests / Load tox environments	9s
allowed-fail-tests / py311-devdeps (ubuntu-latest)	2m 5s
tests / py312 (ubuntu-latest)	5m 10s
tests / py311 (ubuntu-latest)	4m 11s
tests / py311-online (windows-latest)	8m 2s
tests / py310 (macos-latest)	6m 8s
tests / py310-oldestdeps (ubuntu-latest)	4m 34s
docs / build_docs-notebooks (ubuntu-latest)	11m 27s
publish	0s
notify	0s
	42m 13s

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