Implemented c++ iterators for our scene graph attributes.

We only needed const iterators as we don't allow modifying
the scene graph attrs from the iterators.
At iteration time we are only reading or generating attributes on the fly.
The time to influence the attributes which are produced is
before attr generation. This is done by modifying the parameters
on the compute spokes which will generate appropriate scenegraph
functors to reflect your desired settings.

We used the boost iterator facade as this seemed easiest.
We would like to expose the concept of iterators to python as well.