Refactor VaryingCelestialTransform models #882

Job	Run time
sdist_verify	15s
allowed-fail-tests / Load tox environments	11s
docs / Load tox environments	5s
tests / Load tox environments	5s
docs / build_docs-notebooks (ubuntu-latest)	9m 40s
tests / py312 (ubuntu-latest)	5m 20s
tests / py311 (ubuntu-latest)	3m 53s
tests / py311-online (windows-latest)	8m 16s
tests / py310 (macos-latest)	6m 21s
tests / py310-oldestdeps (ubuntu-latest)	4m 41s
allowed-fail-tests / py311-devdeps (ubuntu-latest)	2m 28s
publish	0s
notify	0s
	41m 15s

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