This updates the BFS, DFS & Topological Sort algorithms

DFS had an empty space so I've added the missing content
BFS & DFS now take visitors, like Dijkstra & Bellman-Ford. I've added additional visitor events for DFS to match those used by BGL.
I've added multi-source overloads for DFS & Topological Sort.
General editing to improve the descriptions.

I don't believe the algorithm interfaces are impacted by the addition of descriptors, but that's something to discuss.