This fixes #188

Now, if either vertex or edge filtering is enabled, simultaneous
filtering is activated. By not considering vertex or edge filtering in
completely independent fashion, the compile time memory requirements
reduced to about 50%.

Dumping boost::adjacency_list<> improves memory usage by a factor of
two, and also slightly improves performance in some cases.

This implements a shortest_path() function which returns the shortest
path between two vertices.

This also modifies shortest_distances, such that a predecessor map is
optionally returned.

This includes the shortest_distance() function in the topology module.