OPT: moved 2 functions from sct_register_multimodal to msct_image
OPT: moved 2 functions from sct_register_multimodal to msct_image

Former-commit-id: e2d2eafc6d2fdc90693e9020f3c749fc786df40c
Former-commit-id: db7622ad69aa995fc1481ef7bf76bbec4af87736
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox