OPT: merge_images: reintroduced flag for interpolation
OPT: merge_images: reintroduced flag for interpolation

Former-commit-id: 059f5407ed7730c677c16d01e114440798934748
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox