BUG: fixed bug with CSF segmentation. Fixed #674 and #242
BUG: fixed bug with CSF segmentation. Fixed #674 and #242

Former-commit-id: 5a3d68f8e9609fe501bf6b6d7436910a36dbd82c
Former-commit-id: 2e8a64b3e85c4782b88729efd967589803938620
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox