OPT: now separating first and second registration algorithms in sct_register_multimodal
OPT: now separating first and second registration algorithms in sct_register_multimodal

Former-commit-id: 8c4f2f6fa18853bde443b850b99fc8cb62ec592f
Former-commit-id: b6519a316fc6e92617dfcdd23bda9165e4fc24b4
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox