Add graph conversion

[Boarders]

When we re-implement the topological representation we should add the ability to convert between our graph format and others for example:

• algebraic-graphs
• fgl
• A handwritten binary tree format.

This will allow us to use these formats as intermediate representations for pretty printing/ serialisation amongst other things.

[Boarders]

After experimenting with algebraic-graphs for the network documentation I think it is an extremely bad fit for us given that much of the time we wish to assume our graph is rooted/directed and this fits badly with the graph combinators they define for example it is not even straightforward to write a branch function with something like the following type signature:

 branch :: Graph a -> Graph a -> a -> Graph a 

I'm not sure if fgl fairs any better but it may be worth abandoning this suggestion.