Re #6192. Fix for non 1-1 spectra-detector mapping in padSpectra.

Code taken from the former SpectraAxis 'constructor from a SpectraDetectorMap'.
This correctly handles ISIS instruments where multiple detectors contribute
to a spectrum. This code will hopefully only be temporary pending the elimination
of the SpectraDetectorMap (in #6191).
Merge branch 'feature/6192_remove_spectrum_no_duplication' into develop