detect and error on flow control cycles

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If you construct a fan.In that feeds a fan.Out which in turn feeds that same fan.In (even if this is indirected through a number of intermediary Tubes) it will create a flow graph that will be permanently paused, never passing any traffic. This is an inscrutable failure mode. It would be nice to see immediately that you have set up a cycle and get an exception.

It might be interesting to implement this in terms of a general-purpose interface for getting topology information from a Fount or Drain (maybe a Node interface with two interfaces: "downstreams" and "upstreams"). This sort of interface could also be used for visualizations (graphviz here we come)