OPT: updated sct_warp_template.py concerning spinal levels warping
OPT: updated sct_warp_template.py concerning spinal levels warping

Former-commit-id: 6a51156cd62f10f8571c81a5803f58d8654b5b23
Former-commit-id: e09488a45cd420cd44917619a5ddc1b28bd6a318
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox