'Create a complete(CourseUserGroup + CourseCohort) object.
Args:
cohort_name: Name of the cohort to be created
course_id: Course Id
course_user_group: CourseUserGroup
assignment_type: \'random\' or \'manual\''

@classmethod
def create(cls, cohort_name=None, course_id=None, course_user_group=None, assignment_type=MANUAL):

    if (course_user_group is None):
        (course_user_group, __) = CourseUserGroup.create(cohort_name, course_id)
    (course_cohort, __) = cls.objects.get_or_create(course_user_group=course_user_group, defaults={'assignment_type': assignment_type})
    return course_cohort
