OPT: better way to inverse back wm seg to gm seg
OPT: better way to inverse back wm seg to gm seg

Former-commit-id: b9c7850d4d49fb34c7754669055f04981b00e1d7 [formerly 7271e3c6f7863d325706b8c90de3ab32f2933854]
Former-commit-id: 4139f67bdd103df99f21955e274b780dd7adabbf
Former-commit-id: 43ee337445f3a6be115ce898526383fe91d41893
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox