Allow parallel I/O for hydro and particle reading #7381

Job	Run time
Image tests	11m 21s
unit tests: py3.11 on macos-latest (pytest)	21m 23s
unit tests: py3.11 on ubuntu-latest (pytest)	14m 8s
unit tests: py3.12 on ubuntu-latest (pytest)	11m 20s
answer tests: py3.9 on ubuntu-latest (nose)	16m 12s
unit tests: py3.9 on ubuntu-latest (nose)	22m 20s
unit tests: py3.9 on ubuntu-latest (pytest)	15m 19s
unit tests: py3.11 on windows-latest (pytest)	23m 21s
	2h 15m 24s

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