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.
One reason why they can be useful to detect cases when someone creates, let's say a "merge" vertex, but forgets to wire it up. If it would be a simple case class (which currently is) then this is impossible to detect. However if this method call puts the vertex immediately in a "registry" then we can check if they were connected or not.