MAKE_LABELS - creates a label vector of integers for the input 
  cell array of string
    [LABELS, UNIQUE_LABELS]=MAKE_LABELS(INPUT_LABELS) creates a 
    vector of integer labels (LABELS) for the input cell array 
    of strings INPUT_LABELS. UNIQUE_LABELS contains the strings 
    of unique labels of the input cell array.
 
  Copyright 2011 Dimitrios Zeimpekis, Eugenia Maria Kontopoulou, 
                 Efstratios Gallopoulos