Add new mode for POLARIS reduction `pdf_norm` with absolute vanadium normalisation as default

[RichardWaiteSTFC]

Is your feature request related to a problem? Please describe. POLARIS want to distinguish the reduction for instances when the chopper is stopped but they want to do rietveld refinements and when the full PDF normalisation is required (new mode to be added) - long-term this new mode will replace gudrun but for now just change the default vanadium normalisation.