sct_image: add support to do -split with -o
sct_image: add support to do -split with -o


Former-commit-id: 8e9a9675c3f34a680c892aad296ec60eb3914036
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox