let -vertfile as an optional parameter
let -vertfile as an optional parameter

default -vertfile is `label/template/PAM50_levels.nii.gz` (even when flag is not used), if it does not exist, GM segmentation is done without vertebral information (as with -vertfile None)

Fix issue #1172

Former-commit-id: 084bf452bb0b945ebfb4ca12ece9873d441d7d3c
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox