dwinormalise group: Handle population_template scratch
dwinormalise group: Handle population_template scratch

- Restore scratch directory of population_template to be inside that of dwinormalise group.
- Only retain population_template scratch directory if dwinormalise group is instructed to retain its own scratch directory.

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