Use an origin that is easier to compute - preferably 0

ANTs should handle moving images into the correct space (though yoni may have some counter examples), so forcing the left, posterior, inferior corner of the input volume to be at 1.5, 1.5, 1.5 mm RAS means it's tricky to work out how voxel indices map into this space. Setting the origin to zero (and not flipping its data) seems like the best solution to me, though this would be a breaking change in terms of how we interpret the ants transforms.