TEST: register_multimodal: added test for slicewise registration
TEST: register_multimodal: added test for slicewise registration

Former-commit-id: 5570b9689f4528f2f5c706c72dbe8d073adb6cac [formerly f8138d343b5555d7226b323ab402ec35a82765fc]
Former-commit-id: e6a7a63853191ba7ce2a07ab1a1740b6eb7bb25c
Former-commit-id: f86478780e4ad2d862c017e634a78e508b26679e
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox