Importance sampling for GammaMixture support.

[albertpod]

We have (tw @bvdmitri ) spotted an issue with running an important sampling strategy around the GammaMixture node. We proposed this strategy n this paper.

The API for the form constraint has changed i.e. as = randomvar(nmixtures, prod_constraint = ProdGeneric(), form_constraint = SampleListFormConstraint(rng, 1000, LeftProposal())) compared to as = randomvar(nmixtures, prod_strategy = as_prod_strategy, constraint = as_constraint)

[bvdmitri]

• [x] Sampling from GammaShapeRate is broken

[albertpod]

PR #222

[albertpod]

This issue is related to https://github.com/biaslab/ReactiveMP.jl/issues/27; I will sync the end date for this issue accordingly.