prod_pijk: Auxiliary function to fim_nb()
In kelvinyangli/mbmml: Learn Markov blankets using Minimum Message Length

This is an auxiliary function to fim_nb(). It calculates \prod_j p(x_ij|y=k) for a single data point and store it in a vector for future use. It is mainly for speed up the calculation of fim_nb(). Notice that this function is almost the same as p_ijk(), except that this function calculte the product over all input variables x_j.

1	prod_pijk(dataPoint, pars, xIndices, yValue)

`dataPoint`	A single data point in the given data set.
`pars`	A list of MLE of NB parameters.
`xIndices`	A vector of input variables' indices.
`yValue`	The value of the output variable. The current version of mml_nb only takes binar variables, so the possible y values are 0, 1.