Fix up DWI::get_SH_mapping()
Fix up DWI::get_SH_mapping()

This relied on DWI::guess_DW_directions(), which has been removed in
favour of the DWI::Shells interface.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3