allow the detection of the centerline on images with only one axial slice
allow the detection of the centerline on images with only one axial slice


Former-commit-id: 9fc43c5a82fe6557fd9106a9a92cfef44b1062f7
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox