Fix missing method UnitsMap.get_nwb_file (#1227)

Author: rly

src/pynwb/io/misc.py

@@ -46,5 +46,5 @@ def electrodes_column(self, container, manager):
             return ret
         # set the electrode table if it hasn't been set yet
         if ret.target.table is None:
-            ret.target.table = self.get_nwb_file(container).electrodes
+            ret.target.table = container.get_ancestor('NWBFile').electrodes
         return ret