OPT: remove boundaries outliers from centreline. Improves straightening in case of wrong segmentation
OPT: remove boundaries outliers from centreline. Improves straightening in case of wrong segmentation

Former-commit-id: 10496abc63caaeeac54b194c0a277abf019d2731
Former-commit-id: 55256145e56281f34bb380e2f3aab4246536874b
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox