Dirichlet priors
This MR adds a DirichletPriorDict for if you want to sample from a multinomial distribution.
It's a fun (and hopefully useful) application of the conditional priors.
An example of the prior 
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Requires !332 (merged).
This MR adds a DirichletPriorDict for if you want to sample from a multinomial distribution.
It's a fun (and hopefully useful) application of the conditional priors.
An example of the prior .
Requires !332 (merged).