StereoCameraModel could have functions to project back to image space

[2ros0]

There's already this guy:

void StereoCameraModel::projectDisparityTo3d(const cv::Point2d& left_uv_rect, float disparity,
                                             cv::Point3d& xyz) const

I'm thinking...

void StereoCameraModel::reproject3dToDisparity(const cv::Point3d & xyz, float disparity, cv::Point2d & left_uv_rect) const

So basically, get the pixel location back for a point. It's useful when going back and forth between 3D and 2D representations because you're processing some things in 3D and others in 2D. Thoughts?