fixed slicegroups issue with vert_level (issue #1963)
fixed slicegroups issue with vert_level (issue #1963)


Former-commit-id: 43f286791392d9fc61e62ca90a57475a36f46f47
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox