Merge pull request #1610 from drewejohnson/op-prev-res-bug

Keep user-supplied prev_results on operator

Author: paulromano

openmc/deplete/operator.py

@@ -183,7 +183,6 @@ def __init__(self, geometry, settings, chain_file=None, prev_results=None,
                 "or fission-q".format(energy_mode))
         super().__init__(chain_file, fission_q, dilute_initial, prev_results)
         self.round_number = False
-        self.prev_res = None
         self.settings = settings
         self.geometry = geometry
         self.diff_burnable_mats = diff_burnable_mats