void OnTriggerExit2D(Collider2D collider)
 {
     if (collider.attachedRigidbody.TryGetComponent <SimplePart>(out SimplePart sp))
     {
         if (sp == owner)
         {
             return;
         }
         nearby.Remove(sp);
         owner.OnCellPartExitNearby(sp);
     }
 }