code: def course_is_user_registered(self, course, username=None):
          
          if username is None:
              username = self.session_username()
  
          if self.has_staff_rights_on_course(course, username):
              return True
  
          return self._database.aggregations.find_one({"students": username, "courseid": course.get_id()}) is not None
docs: Checks if a user is registered
          :param course: a Course object
          :param username: The username of the user that we want to check. If None, uses self.session_username()
          :return: True if the user is registered, False else