[ENH] Simplify handling of conformal birth times in RAMSES #7365

Job	Run time
Image tests	6m 21s
unit tests: py3.11 on macos-latest (pytest)	25m 35s
unit tests: py3.11 on ubuntu-latest (pytest)	21m 39s
unit tests: py3.12 on ubuntu-latest (pytest)	3m 30s
answer tests: py3.9 on ubuntu-latest (nose)	20m 57s
unit tests: py3.9 on ubuntu-latest (nose)	12m 26s
unit tests: py3.9 on ubuntu-latest (pytest)	3m 35s
unit tests: py3.11 on windows-latest (pytest)	19m 1s
	1h 53m 4s

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