MAKE_CLUSTERS_SINGLE - auxiliary function for the classification 
  algorithms
    CLUSTERS=MAKE_CLUSTERS_SINGLE(LABELS) forms the cluster 
    structure of a single-label collection with document classes 
    defined by LABELS (vector of integers).
 
  Copyright 2011 Dimitrios Zeimpekis, Eugenia Maria Kontopoulou, 
                 Efstratios Gallopoulos