Use the internal comm for the ensemble mpi wrapper (#4715)

Author: JHopeCollins

firedrake/ensemble/ensemble.py

@@ -24,7 +24,7 @@ def _mpi_dispatch(self, *args, **kwargs):
             return func(self, *args, **kwargs)
         else:
             mpicall = getattr(
-                self.ensemble_comm,
+                self._ensemble_comm,
                 func.__name__)
             return mpicall(*args, **kwargs)
     return _mpi_dispatch