exportDot is now working with MPIIO io library. If called with
MPI-com_self or a mono process communicator it output a full undirected
graph without parallel information. If called with many process each
containing part of undirected graph it encapsulate each part in a
subgraph named with the process id. If node ID is unique across process
common nodes are unique among the representation obtain with dot but
connection do exist in between subgraph(process).
Function API is now reacher. User must at least give something to
associate KEYTYPE object to a unique id. It represents the node
id in the graph associated to the key. Optionally user may set a
specific label to each node and a weight label to each edges.

Some new methods have been added to nodeTo class (one avoid exportDot
to becomes a friend function of the class).