Add bi-directional pipeline node

[jetuk]

A helper node that contains two simple links. This would allow specifying a single capacity, cost, etc. for both directions. Max and min flows may need to be constrained by an aggregated node.

The implementation here might be tricky with determining the orientation of the node. I.e. how would it report flow? Is +ve flow left-to-right, and -ve right-to-left? Will it always be obvious which is "left" and "right". Should those slots have different names?