NEW: Slice instance has now 2 arguments per segmentation : WM and GM
NEW: Slice instance has now 2 arguments per segmentation : WM and GM

Former-commit-id: cc8468f11d58cba56c58fdb2606051860948b8b9
Former-commit-id: 9916bad6a1b00bd6bcc7baf8e27858ddbd6784d2
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox