OPT: keep a default param for step one. (related to #1034)
OPT: keep a default param for step one. (related to #1034)

Former-commit-id: 1a9df10d641ea1ac02d6762bbfdfaceb0b959492
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox