fixel2voxel: Fix dimensionality of output image
fixel2voxel: Fix dimensionality of output image

When the operator chosen to map fixel data to a voxel grid results in a single scalar value per voxel, the output image should logically be a 3D image. However, due to use of the fixel format index image as the template, many such images come out as 4D. This change explicitly sets the output image dimensionality for each choice of operator.

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