fix bug in DWI::gen_direction_matrix() due to floating-point rounding error, causing argument of acos() to be outside range (-1,1) and so produced NaN in rare cases
fix bug in DWI::gen_direction_matrix() due to floating-point rounding error, causing argument of acos() to be outside range (-1,1) and so produced NaN in rare cases

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