NEW: GM/WM ratio can be computed by slice or by vertebral level. Fix issue #660.
NEW: GM/WM ratio can be computed by slice or by vertebral level. Fix issue #660.

3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox