NEW: adapt preprocessing of a dictionary dataset to multiple manual segmentations
NEW: adapt preprocessing of a dictionary dataset to multiple manual segmentations

Former-commit-id: 41b80d40bad64014675c42875e0c116474b1deb0
Former-commit-id: 60e97bb930dd614b63eb3b3396dd73ab0d81dacd
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox