Make gridSize in Services pass correct signature

Both need to conform to Point<int> to be accepted by the engine.

lib/service/simulation_service.dart

@@ -19,7 +19,7 @@ class SimulationService {
     _engine.clear();
   }
 
-  void set gridSize(Point size) {
+  void set gridSize(Point<int> size) {
     _engine.engine.gridSize = size;
   }