Null likelihood

This is an initial attempt at adding a test of the samplers by setting the log likelihood to return 0.

This may not go into the unit testing in the end, but will be needed for review tests.

@vivien @gregory.ashton @moritz.huebner @eric.thrane

bilby/core/sampler/cpnest.py

@@ -78,6 +78,8 @@ class Cpnest(NestedSampler):
                   for key in self.search_parameter_keys]
         model = Model(self.search_parameter_keys, bounds)
         out = CPNest(model, **self.kwargs)
+        if self.likelihood.null_likelihood:
+            out.NS.prior_sampling = True
         out.run()
 
         if self.plot: