Add validation function

[schillic]

Add a function validate(X::LazySet) that takes a set and validates if the set is correctly constructed according to internal assumptions. For instance, the constraints of a HPolygon should be sorted, or an HPolytope should be bounded. (For many set types, this would just be the constant function true.)

[mforets]

Otherwise it could be validate(X::LazySet, f::Function), for example validate(X, isbounded) to really check that the set is bounded. Consider for example this use case.