Improve the function-type interface of bfs, dfs, and dijkstra (ticket #96)
- BfsWizard and DfsWizard have run(s), run(s,t), and run() functions,
DijkstraWizard has run(s) and run(s,t) functions.
- Set NodeMap<T> instead of NullMap as PredMap and DistMap in the default
traits classes for the function-type interface.
- Modify the related test files.
- Doc improvements.
- Bug fix in concepts/path.h.

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