NEW: label_utils: create labels at specified vertebral levels (issue #498)
NEW: label_utils: create labels at specified vertebral levels (issue #498)

Former-commit-id: 780e98d7d40ed1ecc60b4b938d75649259d87203
Former-commit-id: 3c7a6b890e7c2dd59e284381d42fbe4fae3e28fe
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox