OPT: msct_parser.py now generate usage based on the option list
OPT: msct_parser.py now generate usage based on the option list

Former-commit-id: eafbffa880d2201e576cf0f645c0326b87a5fcab
Former-commit-id: 8e38c0376b5c64e55a2e48548e85c25a0a973c06
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox