vis.revcorr.mvnpdf_3

  MVNPDF_3 - fit the data to a mvnpdf function

  R = vis.revcorr.mvnpdf_3(X, XDATA)

  Inputs:
   X - parameters for the gaussian [mu_x mu_y mu_t sigma_1 ... sigma_3 scale]
   XDATA - the data points [X Y T]

  Output:
   R - the evaluated function values

  ERROR: this function is not working