DWI::Shells: Tweak shell selection heuristic
DWI::Shells: Tweak shell selection heuristic

This is particularly to handle shells where the reported b-value varies by less than 1.0 between volumes in the shell.
Initially discussed in d1db0541.

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