This is for debugging only, and users should never need to use
this function - use write_quadtree() instead.
write_quadtree() serializes the CppQuadtree object
(note that the underlying C++ object is actually called
QuadtreeWrapper, but it is exposed to R as CppQuadtree) stored
in the ptr slot of Quadtree.
This function, however, serializes only the Quadtree object contained by the
QuadtreeWrapper.
Usage
# S4 method for character,Quadtree
write_quadtree_ptr(x, y)