A BioGraph with meta-nodes

The merged graph allows for a graph with edges that contains a collection of edges. The idea is to have the same thing for nodes. This will extends the features of the vertexContraction class, able to convert a group of nodes as a single node, except that the latter was of the same type as the groups members, and that the grouped nodes are not easily retrievable.

This will open a new range of metabolic graph variants (ex: network of formulas, EC class...)