REF: separated save_plane in two functions : get_slice and save_plane
REF: separated save_plane in two functions : get_slice and save_plane

Former-commit-id: 36b81821974f9b8f5646ea684f2a9ce66a0e9e88 [formerly 27bac7112f9b34c3c6b6a475a46a4a60268c11a7]
Former-commit-id: c92106219c3611ca401d9abf59cec113e8aedf82
Former-commit-id: ed1598ea5d843311b89e5036340d37310c8dac95
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox