MAKE_VAL_INDS - auxiliary function for the classification 
  algorithms
    INDS=MAKE_VAL_INDS(LABELS) constructs an index vector used 
    during the thresholding phase of any classifier for the 
    multi-label collection with document classes defined by 
    LABELS (cell array of vectors of integers).
 
  Copyright 2011 Dimitrios Zeimpekis, Eugenia Maria Kontopoulou, 
                 Efstratios Gallopoulos