Remove dynesty from __getstate__

This came from an external request to make our Dynesty object picklable. Here the external_sampler (which is the dynesty module) is stored in the object and can't be pickled by pickle.

There may be other neater fixes, if people have thoughts, let me know.

bilby/core/sampler/dynesty.py

@@ -121,6 +121,12 @@ class Dynesty(NestedSampler):
         signal.signal(signal.SIGINT, self.write_current_state_and_exit)
         signal.signal(signal.SIGALRM, self.write_current_state_and_exit)
 
+    def __getstate__(self):
+        """ For pickle: remove external_sampler, which can be an unpicklable "module" """
+        state = self.__dict__.copy()
+        del state['external_sampler']
+        return state
+
     @property
     def sampler_function_kwargs(self):
         keys = ['dlogz', 'print_progress', 'print_func', 'maxiter',