Re #6192. Remove python createSpectraAxis methods (in API v1 & v2).

This is in preparation for changes to the SpectraAxis class, which
will make it impossible to create an axis of this type without a
parent workspace.
The removed methods are unused anyway.
Re #6192. Eliminate spectrum no. duplication between Axis & ISpectrum

Achieved by having the SpectraAxis class hold a pointer to it's owning
workspace and going through that back to the spectrum number held by
the ISpectrum when necessary.
Other code and tests need to be fixed after these changes.
Re #6192. Fix code & tests for changes in SpectraAxis class.
Merge branch 'feature/6192_remove_spectrum_no_duplication' into develop