Extend ruleSPMultiplicationAGPN

[albertpod]

This PR adds a rule for multiplication node:

ruleSPMultiplicationAGPN(msg_out::Message{F, Multivariate}, msg_in1::Message{PointMass, Multivariate}, msg_a::Nothing)

    ^
    | a ~ Univariate
       out ~ Multivariate
-->[x]<--
in1 ~ Multivariate{Pointmass}