Now there is no need for a const_cast when calling find_containing_nodes.
Added an insertObject in the LocalServer.

It is now used as the implementation of LocalServer::makePaddle and
LocalServer::makeBall.
The find_containing_nodes now just asks for a Volume rather than an id.

This is because it was just using the volume anyway and there isn't any
technical reason why the volume needs to be in the Quadtree.
Added a function to snap the top of a volume to the top of another.
closestSideFromInside is now mostProtrudingSide with slightly different behavior.
The start of an impulse-based collision response system is implemented.

It only supports ball-wall collisions at this time.