Extensions for the interface of paths

[Peter Kovacs]

1. Path structures have a function nth(int n) for getting the n th arc of the path. I suggest operator[] as an alias for this function.

2. It would be nice to have source() and target() functions for paths. But it raise questions.
   • Is it necessary that all the arcs in a path are directed in the same direction? Or we could store oppositely directed arc in a path as well? How should we define the source/target? Would the source node be equal to gr.source(path.front()) and the target to gr.target(path.back())?
   • Should paths store source and target nodes explicitly? It would make it possible to handle paths of zero length (with a specified starting node).