Minor improvements to Strahler test.
Add BFS test.
Add DFS test and make some minor improvements.

The minor improvements include
* making DFS use the new successorListOf(V) of GraphSearch Algorithm
  which depends on whether the graph is directed or not
* changing the constructor of DFSInfo to use Integer rather than int

We need to look at where neighborIndex and directedNeighborIndex are
used and replace them with successorListOf(V).