This function implements  the Motion History Image (MHI) technique [1-2], 
  and considers a pixel as in activity, where it should have an 
  absolute intensity jump superior to U.  

  References:
  [1]  Davis, J.W., 'Hierarchical motion history images for recognizing human 
       motion,' Detection and Recognition of Events in Video, 2001. Proceedings. 
       IEEE Workshop on , vol., no., pp.39,46, 2001. doi: 10.1109/EVENT.2001.938864
  [2]  R.P. Godinho, M.M. Silva, J.R. Nozela, R.A. Braga, 'Online biospeckle assessment
       without loss of definition and resolution by motion history image', 
       Optics and Lasers in Engineering, Volume 50, Issue 3, March 2012, Pages 366-372,
       ISSN 0143-8166, http://dx.doi.org/10.1016/j.optlaseng.2011.10.023.

  After starting the main routine just type the following command at the
  prompt:
  graphmhi(DATA,U);
  graphmhi(DATA,U,'off');
  
  Input:
  DATA     is the speckle data pack. Where DATA is a 3D matrix created grouping NTIMES 
           intensity matrices with NLIN lines and NCOL columns. When N=size(DATA), then
           N(1,1) represents NLIN and
           N(1,2) represents NCOL and
           N(1,3) represents NTIMES.
  U        is the activity threshold. Only considered as activity, it changes the 
           intensity values larger than U.
  SHOW     [Optional] If SHOW is equal to string 'off' then not plot the result.
           By default 'on'.

  Output:
  MHI      is the motion history image matrix of data pack. The elements, in 
           the matrix, with higher values pertain to most recent activities


  For help, bug reports and feature suggestions, please visit:
  http://nongnu.org/bsltl

Package: bsltl