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

[carlos-adir]

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

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