UNIQUE_ELEMENTS - detects all distinct elements of a vector
    [ELEMENTS, N] = UNIQUE_ELEMENTS(X) returns in ELEMENTS all 
    distinct elements of vector X, and in N the number of times 
    each element appears in X. A value is repeated if it appears 
    in non-consecutive elements. For no repetitive elements sort 
    the input vector.
 
  Copyright 2011 Dimitrios Zeimpekis, Eugenia Maria Kontopoulou, 
                 Efstratios Gallopoulos