Add multiplication of two Gaussian messages

[HoangMHNguyen]

Update rule for messages has been derived, but the backward messages are somewhat unstable. I'll investigate it a bit more before add this rule to ReactiveMP

[albertpod]

Cool @HoangMHNguyen , once it's done, I will add variational approximation for the multiplication node.

[bvdmitri]

@HoangMHNguyen perhaps you can use hyper-parameters (meta object) to message update rules if it helps with stability in different situations