NEW: average GM and WM in a static method to be more flexible
NEW: average GM and WM in a static method to be more flexible

Former-commit-id: 5c81608ada8525c88c774ce430655304383288fa
Former-commit-id: 5e75561b6f5d8a789d0ff8a9898cf4314803a253
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox