OPT: allow to select only one slice (e.g.: -z 3) (fix issue #858)
OPT: allow to select only one slice (e.g.: -z 3) (fix issue #858)

Former-commit-id: c3c86b93c3d6f284e85136a7d14f5368a0423576
Former-commit-id: f8282fb6581686e35c10caca4bb4f81a4edb230e
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox