move parking structures to a sub-field of system

schema.graphqls

@@ -11,7 +11,11 @@ type System {
   shuttles: [Shuttle!]
   shuttle(id: ID): Shuttle
 
-  # TODO: Implement these in system resolvers
+  parkingSystem: ParkingSystem
+}
+
+type ParkingSystem {
+  systemId: ID!
   parkingStructures: [ParkingStructure!]
   parkingStructure(id: ID): ParkingStructure
 }