NEW: compute GM/WM ratio after segmentation (optional)
NEW: compute GM/WM ratio after segmentation (optional)

Former-commit-id: 32fa7148ce96d6b017e9e860c4bd455b24ae6f95
Former-commit-id: c9d9b7307f16f3c854347172b7394067e33e8034
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox