NNMF_MUL_UPDATE - Applies the multiplicative update algorithm 
  of Lee and Seung. 
    NNMF_MUL_UPDATE applies the multiplicative update algorithm 
    of Lee and Seung for non-negative factorizations. [W, H, S] 
    = nnmf_mul_update(A, W, H, NIT, DSP) produces a non-negative 
    factorization of A, W*H, using as initial factors W and H, 
    applying NIT iterations. 
 
    REFERENCES: 
    [1] D. Lee, S. Seung, Algorithms for Non-negative Matrix 
    Factorization, NIPS (2000), 556-562.
 
  Copyright 2011 Dimitrios Zeimpekis, Eugenia Maria Kontopoulou, 
                 Efstratios Gallopoulos