Add CORE-specific datetime for equity payments

in case payments are added to CORE on a later date, want to keep
original timestamp separate

rattail_corepos/importing/model.py

@@ -90,6 +90,7 @@ class MemberEquityPaymentImporter(importing.model.MemberEquityPaymentImporter):
             'corepos_transaction_number',
             'corepos_transaction_id',
             'corepos_department_number',
+            'corepos_datetime',
         ],
     }