GRMERGEMULTIPLENODES Simplify a graph by merging multiple nodes.

   OUTPUT = grMergeMultipleNodes(INPUT);
   simplify the graph INPUT, and return the result in the graph OUTPUT.
   format for input can be one of
   nodes, edges

   Two steps in the procedure :
   * first remove multiple nodes. find all nodes with same coords, and
       keep only one
   * remove edges that link same nodes

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   author : David Legland 
   INRA - TPV URPOI - BIA IMASTE
   created the 09/08/2004.

Package: matgeom