Include A* PathFinder and not just Dijkstra

[Faron Bracy]

As originally reported by flend

Implement A* as well as Djikstra. The algorithms are very similar, you just need to add a heuristic to the priority queue insertion at line 73 in DijkstraShortestPath. However, you will need to extend EdgeWeightedDigraph so that there is the concept of the grid coordinates of each node, so A* can know what the 'right' direction to go in.