Implemenet Mutual Information, and Joint PDF.
Implemenet Mutual Information, and Joint PDF.

The internal function _column_value_probability now takes a query
Q which can be a vector, and returns the joint density. The API
function column_value_probability is univariate and invokes the
internal one by passing in just [(colno, val)] as the query.

Mutual information is implemented by using simulate, where the
query is the joint distribution of (colno0, colno1). We then use
the new internal _column_value_probability to compute p((colno0,s0))
p((colno1,v1)) and p((colno0,v0),(colno1,v1)) where the v0 and v1
are taken from the simulated sample.

probcomp/bdbcontrib,probcomp/bdbcontrib