Prototype of FlowGraph

Early prototype for discussing the way forward.
Before we start with the DSL sugar I would like that we express the functionality in terms of something similar to the raw addEdge methods.
I agree. I think we could make the `UndefinedSource` and `UndefinedSink` the tokens. (these are not real Source/Sink)
Perhaps including a string name, allowing for lookup in the `PartialFlowGraph`.
I will give it a try.