REF: simpler name for the warped grid (qc for register_graymatter)
REF: simpler name for the warped grid (qc for register_graymatter)

Former-commit-id: 50e55cd29219168950eefd689434886fb8d61695 [formerly eb120787909285bbbcb843d440a7b72e03e52496]
Former-commit-id: 06c5f6f314a600b94c4bc156cf79eae3cd6fa971
Former-commit-id: c71b2d609e25f466890ae5aaa1c8fd728920dccf
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox