Small fixes to DiskChopper.from_nexus #764

Job	Run time
Formatting and static analysis	15s
Mantid tests	1m 44s
Tests (ubuntu-24.04, 3.11, py311) / package-test	24s
Tests (ubuntu-24.04, 3.11, py311) / test	2m 54s
docs / Build documentation	5m 36s
	10m 53s

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