Add bills and piggy banks

app/Repositories/ObjectGroup/ObjectGroupRepository.php

@@ -163,6 +163,14 @@ class ObjectGroupRepository implements ObjectGroupRepositoryInterface
         return $objectGroup->piggyBanks;
     }
 
+    /**
+     * @inheritDoc
+     */
+    public function getBills(ObjectGroup $objectGroup): Collection
+    {
+        return $objectGroup->bills;
+    }
+
     /**
      * @inheritDoc
      */