sct_image: added a condition on squeeze data
sct_image: added a condition on squeeze data

need to be singleton


Former-commit-id: 639d718add3fd733d5cdcb519947bfee941a9ae2
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox