Is AD_HDF5_SINGLE for general use?

[prjemian]

Via conversations on Slack, it seems the AD_HDF5_SINGLE handler is specific to some HDF5 files generated by the SIX instrument at facility NSLS-II. If so, that's too bad since the name suggests a more general use.

Maybe acknowledge (by stating boldly in the documentation) that AD_HDF5_SINGLE is only for NSLS-II instrument SIX. Make a new handler (say AD_HDF5_GENERAL) which matches the files generated via FileStoreHDF5SingleIterativeWrite (which may need some modification as well). Then, if SIX still uses this mixin, they, alone, would need to customize for their specific use.

One issue noted by @tacaswell with the AD_HDF5_SINGLE handler:

that handler does not conform to what we expect handlers to do (return an ND sliceable thing)

Related to: BCDA-APS/apstools#691