NEW: sct_register_graymatter output WM and GM segmentations. Fixed #535
NEW: sct_register_graymatter output WM and GM segmentations. Fixed #535

Former-commit-id: d667764e86033b1c9b7b7b19e4f65265a66b75db
Former-commit-id: 3d3b5e4acdec5b9c4e81f4338485376a03e735c1
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox