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>

 Document: lemon-refman

 Title: LEMON Reference Manual

 Author: LEMON Development Team

 Abstract: Documentation of LEMON.

 Section: unknown

 Format: HTML

 Index: /usr/share/doc/lemon/html/index.html

 Files: /usr/share/doc/lemon/html/*.html