Feature: Add __bool__ operators for `Empty` and `Whole`

To check if two subsets are disjoint, one can do:

```python
if subseta & subsetb == Empty():
    print("They are disjoint")
else:
    print("They are not disjoint")
```

The intention is to add `__bool__` operator in `Empty` and `Whole`, since they present the meaning of false and true.

Not yet sure if the other classes such as `SingleValue` or `Interval` or `Disjoint` should give as result