group: Two fixes
group: Two fixes

- Fix N=1 intensity normalisation factor using undefined variable due to having been duplicated from another area of code.
- Defer calculation of voxel volume from the SessionPaths class constructor to later in execution, so that the scratch directory will have already been constructed, and therefore the Image.Header constructor will write the corresponding temporary JSON to the scratch directory, avoiding a filesystem write access issue in some circumstances.

BIDS-Apps/MRtrix3_connectome