OPT: register_multimodal: now possible to do multiresolution and specify gradStep for algo=slicereg
OPT: register_multimodal: now possible to do multiresolution and specify gradStep for algo=slicereg

Former-commit-id: aff3920d46b43e250c14d99ecbd1d672d4ae035d
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox