Allow CPO edges(g, u) to be also customized by u.edges() #175

Currently, CPO edges(g, u) is considered valid when (among other options) u.edges(g) is valid. But consider this implementation of a graph:

namespace MyLibrary 
{
  struct MyEdge {
    string content;
    int indexOfTarget;
  };

  struct MyVertex {
    string content;
    vector<MyEdge> outEdges;
  };

  class MyGraphA {
    std::vector<MyVertex> _vertices;
  };

  class MyGraphB {
    std::vector<MyVertex> _vertices;
  };
}

At the point of defining my class MyVertex I do not know the type of the graph that will be holding me. In fact, more than one graph type may be storing the same vertex type. If MyVertex were to define the member funciton edges, I do not know what graph type to pass it. And on the other hand, MyVertex::edges does not need the graph type for anything. I can of course make MyVertex::edges a template, but why forcing me to declare templates?

Alternatively, extend the customization point edges(g, u) so that it is valid when wither u.edges(g) is valid or u.edges() is valid.