DEV: spinal_level: updated code to generate spinal levels on the PAM50
DEV: spinal_level: updated code to generate spinal levels on the PAM50

Former-commit-id: 53616ab771c8a649cfc8d6f0f68ee3abd21fa707
Former-commit-id: f64422b7d105319b1beb4994b3a6b67f1bc0d83a
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox