Fixes to dwibiascorrect script
Fixes to dwibiascorrect script

- Try to identify the suffix that FAST will use for its output, so that the subsequent command can find it.
- Test input image to make sure it is a valid DWI, rather than relying on dwi2mask.

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