FormatReader: clear the 'core' variable

Suggested by Curtis Rueden.
Zeiss CZI: tidy up metadata hashtable keys
Fix the Imaris metadata page
Make sure that SizeZ/SizeC/SizeT are set for single-file exports
Deltavision: make sure that the DetectorSettings.ID is set

See QA #4877.
Metamorph: look for the channel count in one more place

Reported by Stéphane Dallongeville.