Fix 'at' property for transfergroups

kfet/management/commands/createopes.py

@@ -147,7 +147,6 @@ class Command(BaseCommand):
                 comment = ""
 
             transfergroup = TransferGroup(
-                at=at,
                 comment=comment,
                 valid_by=random.choice(accounts)
             )
@@ -164,6 +163,9 @@ class Command(BaseCommand):
                 transfer.save()
                 transfers += 1
 
+            transfergroup.at = at
+            transfergroup.save()
+
         self.stdout.write(
             "- {:d} opérations créées dont {:d} commandes d'articles"
             .format(opes_created, purchases))