NEW: new version of sct_segment_graymatter using asman's method (work in progress)
NEW: new version of sct_segment_graymatter using asman's method (work in progress)

Former-commit-id: 7309621d44105aac3cbb679edd6d1a47631807b2
Former-commit-id: f3ece1205484bcd81ed422d2fa0f20497a2e6cbc
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox