OPT: binarise added in segment_graymatter
OPT: binarise added in segment_graymatter

Former-commit-id: ccc985fd623fd48ad6b2be1d4e96454967d3d6ca
Former-commit-id: f19c5631705a66bdae474cdd66dc218275a2c967
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox