OPT: detect_vertebral_levels: simplified code-- only tested for T1
OPT: detect_vertebral_levels: simplified code-- only tested for T1

Former-commit-id: 5445b029ba44032dc42de95412bf751eab1137da [formerly 73d2284cd1e4d0548f494a026107137555fb8d7b]
Former-commit-id: 5d8f1cdfabf7b93eaa6865139edc9e62d28e1703
Former-commit-id: 5070336cdface3359615d6abc7859a2fc78663ef
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox