OPT: sct_register_multimodal now using parser and more intuitive flags
OPT: sct_register_multimodal now using parser and more intuitive flags

Former-commit-id: 566727ed0fa85255f9eca8f121bb4fca70a42ccd
Former-commit-id: bcc9f179e9acc8e757d0d40b8233cc7699a4c549
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox