Fix opposite idempotency

When IDEMPOTENCY is set to error, the following code will throw an error "The Propagator solver.constraints.reification.PropOpposite@9 is not idempotent!"

```java
public static void main(String[] args) throws Exception {
    Solver s= new Solver();
    s.post(SCF.all_equal(
            new SetVar[]{VF.set("s1", 0, 3, s), VF.set("s2", 0, 3, s)}).getOpposite());
    System.out.println(s.findSolution());
}
```

Same issue with PropElementV_fast. (Note that PropElementV_fast is still not idempotent, but with this change, less errors will be thrown.)