OPT: detect_vertebral_levels: added output flag
OPT: detect_vertebral_levels: added output flag

Former-commit-id: 051a37132d78b3e13e5098a8facefdd7325f8b66
Former-commit-id: e25bc66aea4039102e1e3f51c0040c5e3740382b
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox