OPT: allow for no arguments in parser when no mandatory options
OPT: allow for no arguments in parser when no mandatory options

Former-commit-id: 16e99e2366aaf06bbb9721edb15194ef754b6803
Former-commit-id: 0575e0cc3ea3122208a03b752f2c81eeeeef4648
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox