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.
if Flow had a name then people could use this to hand out PartialFlowGraphs as a single package reusable piece; it would be of course more type-safe to include the actual ins and outs but there might be people who would forego that …