Re #6191. Method to update detector IDs from the new mapping class.

Takes in a SpectrumDetectorMapping objects and goes through the
ISpectrum objects updating the detector IDs according to the
previously set spectrum numbers.
Re #6191. Use new workspace method to update detector IDs.
Re #6191. LoadRaw no longer touches the SpectraDetectorMap at all.

Instead it uses the SpectrumDetectorMapping class where necessary.
There may be some redundant setting of the detector IDs but this thing
is too complex to mess with more than necessary.