OPT: detect_vertebral_levels: improve correlation at the edge
OPT: detect_vertebral_levels: improve correlation at the edge

Former-commit-id: 6d6498a0172175c9e14ca97b653aae4d81bc4677
Former-commit-id: e13cf3ce45abe7182487b26d8aaf1ce5a307e356
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox