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>

 /*!

 \page ugraphs Undirected graph structures

 The primary data structures of LEMON are the graph classes.

 Under construction.

 */