Bayesian model averaging

[drbenvincent]

Suggestion from @ricardoV94: In situations where there are multiple valid models, then we either have to pick what model we want to use, or we can do Bayesian model averaging. So you can just fit both model, do model comparison which gives the model weightings, then we can generate model averaged predictions.

I think this was done as posterior_predictive_w (or similar) in PyMC3, but was not ported to v4.