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 .

Requires !332 (merged).

bilby/core/prior/dict.py

 from importlib import import_module
 from io import open as ioopen
 import json
-# import numpy as np
+import numpy as np
 import os
 
 from future.utils import iteritems