Small fixes to DiskChopper.from_nexus #779

Job	Run time
Formatting and static analysis	17s
Mantid tests	1m 48s
Tests (ubuntu-24.04, 3.11, py311) / test	2m 37s
Tests (ubuntu-24.04, 3.11, py311) / package-test	20s
docs / Build documentation	5m 58s
	11m 0s

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