Update shortest_path_heuristic.hpp

[TachikakaMin]

Some improvement when building our distance graph.

[whoenig]

Could you describe what the improvement is? It seems to me like there might be some runtime benefit (direct lookup, rather than hash-map check), although both operations are O(1). Nevertheless, this does come at the cost of memory overhead, since now a large 2D array is needed. Any numbers would be helpful to decide if this should be merged.

[TachikakaMin]

Yes, you are right. I didn't notice it is an unordered map.