Add support for `iterate_over_all` for the CombinedDataset #634

Job	Run time
pytest (ubuntu-latest, 3.9, oldest)	2m 44s
pytest (ubuntu-latest, 3.9, latest)	1m 58s
pytest (macos-13, 3.9, oldest)	6m 41s
pytest (macos-13, 3.9, latest)	7m 10s
pytest (windows-latest, 3.9, oldest)	5m 13s
pytest (windows-latest, 3.9, latest)	5m 25s
	29m 11s

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