fixelfilter: Test input as directory rather than file first
fixelfilter: Test input as directory rather than file first

This prevents the appearance of a message attempting to load a fixel directory as a DICOM image.

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