RFE: support sub-graphs

[ankostis]

It would be nice to add e method like:

bigger_graph = Graph.add_subgraph(some_graph)

and port all nodes from some_graph into bigger_graph.

[cedricleroy]

I once thought about that, and kinda liked the idea. The big PROS back in the times was that it is somehow easier to design small graphs rather than big ones. I then thought the idea was actually just hiding a bigger problem between data and graph modeling which could be solved with dynamic nodes (nodes created at run-time depending on the inputs passed to the graph).

Do you have an illustration on your thoughts for the sub-graphs idea?