extract_metric: perslice kind of working, but need to fix various extensions output
extract_metric: perslice kind of working, but need to fix various extensions output


Former-commit-id: c122a19f9ef73397baf695cc14221366ecd424cf
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox