OPT: add default value for the vertebral labelling file (./label/template/MNI-Poly-AMU_level.nii.gz ) and check its existence (in case vertebral levels are selected)
OPT: add default value for the vertebral labelling file (./label/template/MNI-Poly-AMU_level.nii.gz ) and check its existence (in case vertebral levels are selected)

Former-commit-id: 8df77d8ad276ce71179d3d7c7ed280a9be151850
Former-commit-id: 09efd416993a225c07ae054e3a2b8cfdc01d07da
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox