OPT_2MEANS - a special case of k-means for k=2
    OPT_2MEANS(A, X) returns the clustering that optimizes the 
    objective function of the k-means algorithm based on the 
    ordering of vector X.
    [CLUSTERS, S]=OPT_2MEANS(A, X) returns the cluster 
    structure as well as the value of the objective function. 
 
  Copyright 2011 Dimitrios Zeimpekis, Eugenia Maria Kontopoulou, 
                 Efstratios Gallopoulos