Added property 'visible' for cohorts. Unsetting it hides the cohort from users in the course.
AMOS BEGIN
CPY [visible,core_moodle],[visible,core_cohort]
AMOS END
PARAM_NUMBER is a float, our id's are int's
* Fix core_cohort_get_cohorts to specify correct return type on cohort id
* Fix core_enrol_get_enrolled_users_with_capability to specify correct
correct return type on user id
* Fix core_cohort_update_cohorts to specify the right parameter type
(int) on cohort id
* Add tests to verify the bug on core_cohort_update_cohorts param type.