OPT: no default is given for the vertebral labelling file anymore and a check is performed if the user asked for vertebral levels but didn't specified any vertebral labeling file (issue #810)
OPT: no default is given for the vertebral labelling file anymore and a check is performed if the user asked for vertebral levels but didn't specified any vertebral labeling file (issue #810)

Former-commit-id: cd669e2c7749523e0a4848ed57df06f01804c0e4 [formerly 4d89225c50f15b4ca1f71310c420f7f46603d249]
Former-commit-id: cf663b1e9f45fa03e7605174b4ffb5427a5aeb95
Former-commit-id: 823a3218594e232f0d8e94c70e0099bedab68b88
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox