mrinfo: add option '-validate' to have DW scheme processed
mrinfo: add option '-validate' to have DW scheme processed

This option means the DW scheme would be imported using
DWI::get_valid_DW_scheme(), which means the DW scheme would be sanitised
as it would otherwise be in any other DWI-related MRtrix app, and the
-bvalue_scaling option would be honoured. This should sort out the
remaining issues needed to address #178.

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