fix(groups): exception now throws when one tries to add the same entity to two groups

Author: CGjupoulton

engine/modules/entities/src/main/java/com/codingame/gameengine/module/entities/ContainerBasedEntity.java

@@ -51,6 +51,7 @@ public void add(Entity<?>... entities) {
                 throw new IllegalArgumentException();
             }
             this.entities.add(entity);
+            entity.parent = this;
         });
 
         set("children", asString(this.entities), null);