fix default list in dict

sandbox/engulf/simulation.py

         super().append(p_object)
 
         if isinstance(p_object, Cell):
-            self.cell_xyz.get(p_object.position, []).append(p_object)
+            self.cell_xyz.setdefault(p_object.position, []).append(p_object)
 
         if isinstance(p_object, Grass):
-            self.grass_xyz.get(p_object.position, []).append(p_object)
+            self.grass_xyz.setdefault(p_object.position, []).append(p_object)
 
 
 class Engulf(XYZSimulation):

synergine2/xyz.py

                 str(p_object),
             ))
 
-        self.xyz.get(p_object.position, []).append(p_object)
+        self.xyz.setdefault(p_object.position, []).append(p_object)
 
 
 class XYZSimulation(BaseSimulation):