dev/spinal_level/create_PAM50_label_spinal_level.py: add usage
dev/spinal_level/create_PAM50_label_spinal_level.py: add usage


Former-commit-id: e3d41f7b7b843f2157f7e96b3fef8e818cdf2772
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox