Use MSGReducer for OSIRIS diffspec mode. Refs #6791

Reorganised OSIRIS parameter files to follow IRIS/TOSCA. The interface
now uses the same MSGReducer for diffspec mode and only uses the workflow
algorithm for diffonly mode.
Remove DiffSpec mode from OSIRISDiffractionReduction.

Leave it doing diffraction only in favour of MSGReducer covering
spectroscopy mode. Refs #6791