simple_predictive_probability should evaluate density for joint distribution

[fsaad]

There is currently no way to evaluate P(X=x,Y=y,Z=z) under the joint density of (X,Y,Z). We have

    def simple_predictive_probability(self, M_c, X_L, X_D, Y, Q):
        :param Q: A list of values to sample.  Each value is doublet of (r, d, v):
                  r is the row index, d is the column index, v is the value
        :type Q: list of lists
        :returns: list of floats -- probabilities of the values specified by Q

which can only evaluate P(col_d = v | row = r), or the univariate marginal distribution for col_d. If you input multiple columns, than multiple univariate densities are returned. We need Q to take a list of lists of tuples.

[fsaad]

Duplicate #62 -- kept that version because of vkm comment while this is bland.