ENH: Implement general sum-product function for Node
ENH: Implement general sum-product function for Node

This function computes the product of a set of arrays, sums over the
plates that should not be in the result and applies correction term if
some plates are broadcasted.

jluttine/bayespy,bayespy/bayespy,fivejjs/bayespy,SalemAmeen/bayespy