Access the topology of a graph

[Quincunx271]

Given a graph that the user constructed, I need to be able to access the topology of the graph.

Currently, it almost seems possible with FlowScene::nodes() and FlowScene::connections(), but although Connection is defined through its header, Node is not. Node is merely forward declared. If Node.hpp was visible or #include'd somewhere, it would work.

[russelltg]

Agreed. On my fork I actually export Node and include <../src/Node.hpp>. It's definitely a needed feature.