pymultinest.py: set n_params to ndims

 - attempt to resolve lscsoft/bilby#459 for PyMultiNest >= 2.9

bilby/core/sampler/pymultinest.py

@@ -81,6 +81,10 @@ class Pymultinest(NestedSampler):
                 .format(self.kwargs['outputfiles_basename']))
         check_directory_exists_and_if_not_mkdir(
             self.kwargs['outputfiles_basename'])
+
+        # for PyMultiNest >=2.9 the n_params kwarg cannot be None
+        if self.kwargs["n_params"] == None:
+            self.kwargs["n_params"] = self.ndim
         NestedSampler._verify_kwargs_against_default_kwargs(self)
 
     def _apply_multinest_boundaries(self):