Small bug fix in dwi_extract when the number of b0's or DWI's is 1
Small bug fix in dwi_extract when the number of b0's or DWI's is 1

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