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]

• Sampling from GammaShapeRate is broken

[albertpod]

PR #222

[albertpod]

This issue is related to #27; I will sync the end date for this issue accordingly.

[albertpod]

Just a self reminder that this needs to be checked. I don't think that #27 fixed this particular issue.