OPT: changed default z gap from 2 to 15
OPT: changed default z gap from 2 to 15

Former-commit-id: 7dc5f22fd261a41b9d009cc34ca4eecdceb169af
Former-commit-id: 1702cf7001bb1238404cabfe924224b40eeee772
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox