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