HDX/AV: Movement of people or goods on a highway network

[jteresco]

This is an idea for an application that could be implemented as an AV that takes full advantage of the graph structure. There could be items (such as a population or a quantity of goods or something similar) at each (or at a subset of) waypoints at the start. These items would "flow" from waypoints with larger numbers of items to neighbors with smaller numbers of items via graph connections.

This is related to #142 and possibly a variant on the same AV when that is implemented.