Revising the bpugraph concept

We need a public but very limited ANode and BNode class
It can be used with ItemSetTraits and with some special maps

By example:
DescriptorMap<Graph, ANode>
InvertableMap<Graph, ANode, string>
IterableBoolMap<Graph, ANode>
IterableIntMap<Graph, ANode>
IterableValueMap<Graph, ANode, string>

 /// The namespace of LEMON

 /// \todo Some more detailed description would be nice here.

 ///

 namespace lemon {

   /// The namespace of LEMON concepts and concept checking classes

   /// \todo Some more detailed description would be nice here.

   ///

   namespace concept {}

 }