[xGraph] turn exportDot into a parallel MPIIO export function + API change
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).
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