OPT: changed default gapz to 20vox size for cross generation
OPT: changed default gapz to 20vox size for cross generation

Former-commit-id: 62ecd6b51835cb6f36d8ff4feec17e6cf302ea35
Former-commit-id: 06abfe1c040413c6878de617c3b67bc1dd5bd515
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox