Removed now useless checks as they are checked via traits

Created by: stefanoborini

Fixes #136 (closed) by rephrasing the issue. The point is that we don't need the check as we are guaranteed by initialization that the class will contain something that satisfies the trait, that is, it will not be None due to the allow_none=False option. if the get_ methods are not redefined to return something, they will raise a NotImplementedError which is self-describing. If the trait is assigned something it doesn't like it will also report a meaningful error message.