Allow use of different ultranest step samplers

bilby/core/sampler/ultranest.py

@@ -241,6 +241,7 @@ class Ultranest(NestedSampler):
 
     def run_sampler(self):
         import ultranest
+        import ultranest.stepsampler
 
         if self.kwargs["dlogz"] is None:
             # remove dlogz, so ultranest defaults (which are different for
@@ -249,7 +250,7 @@ class Ultranest(NestedSampler):
 
         self._verify_kwargs_against_default_kwargs()
 
-        stepsampler = self.kwargs.get("step_sampler", None)
+        stepsampler = self.kwargs.pop("step_sampler", None)
 
         temporary_outputfiles_basename = tempfile.TemporaryDirectory().name
         self.temporary_outputfiles_basename = temporary_outputfiles_basename