convert: multi union to fluka #801

Job	Run time
Calculate and upload test coverage	3m 36s
Test with Python (3.9, ubuntu-latest)	3m 38s
Test in Docker container	3m 24s
Test with Python (3.9, macOS-latest)	3m 38s
Test with Python (3.9, windows-latest)	3m 31s
Test with Python (3.10, ubuntu-latest)	3m 38s
Test with Python (3.10, macOS-latest)	3m 39s
Test with Python (3.10, windows-latest)	3m 29s
Test with Python (3.11, ubuntu-latest)	3m 37s
Test with Python (3.11, macOS-latest)	3m 38s
Test with Python (3.11, windows-latest)	3m 33s
Test with Python (3.12, ubuntu-latest)	3m 38s
Test with Python (3.12, macOS-latest)	3m 39s
Test with Python (3.12, windows-latest)	3m 31s
	50m 9s

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