dwipreproc: check correct gradient files. reverted dee7304 #753
dwipreproc: check correct gradient files. reverted dee7304 #753

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