OPT: correction in creation of mask from centerline
OPT: correction in creation of mask from centerline

Former-commit-id: 07de8c51c2bc2d17ebe8beb230add6b4ffeb6dd8 [formerly 7aa87e7c081bb032bb2c77e49e91977033b4c419]
Former-commit-id: ec0741af9cd8ea356c0da87dc35d24c28a17f98d
Former-commit-id: 83559af3f45ac871caa83b516e32233fefbb03d8
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox