MAINT: Use lazy_loader for lazy loading of submodules #577

Job	Run time
Formatting and static analysis	10s
Mantid tests	24s
Tests (ubuntu-24.04, 3.10, py310) / package-test	23s
Tests (ubuntu-24.04, 3.10, py310) / test	2m 33s
docs	0s
	3m 30s

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