More general support for handling of hit indices

[jbeilstenedmands]

Motivated by discussion around #709 , it would be a nice feature to have in general, although it's not clear whether this should be handled at the format class level or as an option in dials.import, or both!

The use case I have in mind is serial data collection with low average hit rate, where some live hit rate analysis has been performed, and those results are available, potentially written into a data array of a h5 file.
A suggestion was made that for Nexus format data, this could be handled by the creation of a virtual dataset containing only the hit images.