DEV: added option to select the gap between points
DEV: added option to select the gap between points

Former-commit-id: e2435d11a15c7e5f2f6782eed6805cd27d892e36
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox