Generalise change notifications in scene-graph

[mattngc]

• Currently only the root is informed of notifications to the scene-graph.
• It would be useful for one node to know when the geometry of another node has changed. i.e. targeted pixel sampling in a physical pinhole camera. This would allow caching of data and faster performance.
• Need to avoid circular changes. i.e. change in one node means change in another node, etc. (This might need to be a warning to the developer in the documentation).