OPT: detect_vertebral_levels: now searches in IS direction (DEBUG MODE)
OPT: detect_vertebral_levels: now searches in IS direction (DEBUG MODE)

Former-commit-id: e45e1f70c2a60a541313b35eb7214675b6731608
Former-commit-id: 98bd30a0673d312d658694aea3ca740780a4950f
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox