Define the User entity. The entity SystemGroup will not be created. Each system will have its own users and a user could be suscribed to more than one system.
Refactor user entity. Add UserDAO class.
Add relationship between systems and users.
Add UserRepository class.
Add UserBusiness class.
Add UserResource class in order to make the user's API available.
Add test for the method 'getAllUsersSubscribedToASystem' from the SystemResource class.
Add test for the class UserDAO.
Add tests for the class UserRepository.
Add tests for the class UserBusiness. Fixes #1.
Add tests for the class UserResource. Fixes #2.