Build a dictionary mapping each pair of nodes to a number (the distance between them).
        
        @type  graph: graph
        @param graph: Graph.

def optimize(self, graph):
        """
        Build a dictionary mapping each pair of nodes to a number (the distance between them).
        
        @type  graph: graph
        @param graph: Graph. 
        """
        for start in graph.nodes():
            for end in graph.nodes():
                for each in graph.node_attributes(start):
                    if (each[0] == 'position'):
                        start_attr = each[1]
                        break
                for each in graph.node_attributes(end):
                    if (each[0] == 'position'):
                        end_attr = each[1]
                        break
                dist = 0
                for i in range(len(start_attr)):
                    dist = dist + (float(start_attr[i]) - float(end_attr[i]))**2
                self.distances[(start,end)] = dist