Revision of the domain definitions

This MR revises the domain definitions. The main points are the following:

-Unified behaviour of Domain.check_domain_consistency and Domain.update_domain. The behaviour of these functions differ between the subclasses Options and Interval.

• Interval supports now the definition of a integer interval. (previously implemented via Options).
• Options with integers, behave nearly the same as Options with any categorical data. The difference lies in the sample around method, as the work on a sorted list of the options.
• Cleaning code in Dataset