OPT: set the dimensionality to 2 when the warping field is 2D. Fix #1419
OPT: set the dimensionality to 2 when the warping field is 2D. Fix #1419

Former-commit-id: ca651cc1625e42ea55a17a98e0e6d9bd7d03d704
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox