OPT: fixed usage (size in voxel instead of mm) and fixed estimation of radius (forgot to add +1 on positive side of matrix)
OPT: fixed usage (size in voxel instead of mm) and fixed estimation of radius (forgot to add +1 on positive side of matrix)

Former-commit-id: a727e2054c7b16f89aa3bb848cf388f9527b059c
Former-commit-id: 121badb8f5054fb06e0f39473bf764422c1eabb9
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox