Fix #2

Basically, let us factorize the density as

p1(sigma) p2(leaf|sigma) p3(d|leaf)

I realized that Gibbs did not take p1 into account. The SMC algorithms
were OK since I am proposing the sigma's from the prior, so this part
just disappears in the weight updates.

After fixing this, it looks like the three methods agree in small
problems.