Depletion restart with mpi (#2778)

Co-authored-by: Paul Romano <[email protected]>

openmc/deplete/coupled_operator.py

@@ -291,7 +291,7 @@ def _load_previous_results(self):
         # on this process
         if comm.size != 1:
             prev_results = self.prev_res
-            self.prev_res = Results()
+            self.prev_res = Results(filename=None)
             mat_indexes = _distribute(range(len(self.burnable_mats)))
             for res_obj in prev_results:
                 new_res = res_obj.distribute(self.local_mats, mat_indexes)