sct_utils: clean-up add_suffix and extract_fname
sct_utils: clean-up add_suffix and extract_fname


Former-commit-id: 2e43f1f7dd9d6a585f7172acf7f0d28b2d9e77b2
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox