fix minor bug whereby 3D mask couldn't be used with 4D image
fix minor bug whereby 3D mask couldn't be used with 4D image

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