Add log barrier to steer apriltag reprojection factor
Reprojection-based factors can let the solver jump to a solution where the camera is behind the tag. We can steer the solver away from this solution by ensuring that the Z axis of the camera and the tag are less than 90° apart (cos > 0). A log barrier with a quadratic part is implemented based on https://arxiv.org/abs/1905.06144 (9).