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.
This is not good: instead of the HasNoSink flow we should base this on a Sink token. This does require that the token is generated outside of the graph DSL, but otherwise we won’t have a unique addressing scheme where this Sink shall go (cf. your comment about the same flow being used multiple times in a graph, which is entirely reasonable; then it is also reasonable to replace the Sink only on one of them).