ContinuousTransition node does not have mean-field rules

[albertpod]

The current implementation of the continuous transition node allows only structured factorization between the states, i.e.,

constraints = @constraints begin
    q(x, y, h, Λ) = q(x, y)q(h)q(Λ)
end;

While this factorization is beneficial for state space models (as it introduces coupling between states), it doesn't allow one of $x$ or $y$ to be observed (clamped). The implementation of these (MF) rules needs to be added, which will also allow the following factorization:

constraints = @constraints begin
    q(x, y, h, Λ) = q(x)q(y)q(h)q(Λ)
end;

[bvdmitri]

Fixed in #406