fixel2voxel: Additional split operations
fixel2voxel: Additional split operations

Can now extract and split up any fixel data: sizes, associated values, and directions.

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