dwi2response fa: Fix
dwi2response fa: Fix

This algorithm was registering as single-shell, which resulted in removing the b=0 data and erroneous tensor estimation.

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