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The edge and adjacency iterators of UndirectedAdaptor are now
implemented using Boost.Iterators and Boost.Range. Among other things,
this preserves underlying properties of the original iterators, such as
random access.

This also modifies the behaviour of the graph_properties dict, which now
return the property values directly, instead of the map objects.

This is to avoid strange segfaults in atexit functions defined by
boost::python.