NEW: new method to generate continuous vertebral levels
NEW: new method to generate continuous vertebral levels

Former-commit-id: c74c72abbb1faa8b58f1e0e4d0f956427f8f709e
Former-commit-id: 4273ad3351902c906ad2063e46d504ffc003c9f8
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox