Safer node requests

Currently the VolumeTreeNode can be queried for a node via getContainingNode(Point const& p) which exposes the ability to traverse node tree too easily and can lead to bugs.

This has been reported by @Nikos to cause issues when a Point outside of the largest volume are requested. In this case a null pointer is returned (expected behavior) but can also be then asked about its properties which is segfaults.

In the future, we should force developers to use always use the particle.getNode() which is always up to date and will keep us from continuously traversing the world tree.