dirgen: optimise all directions, don't force first direction along z axis
dirgen: optimise all directions, don't force first direction along z axis

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