sct_compute_snr: add -r argument
sct_compute_snr: add -r argument

This makes all the scripts balanced. This one was an exception.


Former-commit-id: a9e3d72939f5b13528c509dfae7d283e6d3e50b6
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox