Standard STL read-only container stuff

These just provide access to the point data in a clean way

* \brief Lifts a point from the image plane to its projective ray * * \param p image coordinates * \param P coordinates of the projective ray */

* Returns whether the node is a leaf node * @return true iff the node is a leaf */

Add a point to the spline, and invalidate it so its recalculated on the next access