REF: register_multimodal: forcing shrink=1 when using slicewise=1 with SyN
REF: register_multimodal: forcing shrink=1 when using slicewise=1 with SyN

Former-commit-id: 8db489389b6649b1226fca0cceb5be33920b4c1a
Former-commit-id: 741382120e4b9a1adcf3085705fcf727c557a5ed
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox