Checks if a user is can access a course
        :param course: a Course object
        :param username: The username of the user that we want to check. If None, uses self.session_username()
        :param lti: indicates if the user is currently in a LTI session or not.
            - None to ignore the check
            - True to indicate the user is in a LTI session
            - False to indicate the user is not in a LTI session
            - "auto" to enable the check and take the information from the current session
        :return: True if the user can access the course, False else

def course_is_open_to_user(self, course, username=None, lti=None):
        """
        Checks if a user is can access a course
        :param course: a Course object
        :param username: The username of the user that we want to check. If None, uses self.session_username()
        :param lti: indicates if the user is currently in a LTI session or not.
            - None to ignore the check
            - True to indicate the user is in a LTI session
            - False to indicate the user is not in a LTI session
            - "auto" to enable the check and take the information from the current session
        :return: True if the user can access the course, False else
        """
        if username is None:
            username = self.session_username()
        if lti == "auto":
            lti = self.session_lti_info() is not None

        if self.has_staff_rights_on_course(course, username):
            return True

        if not course.get_accessibility().is_open() or (not self.course_is_user_registered(course, username) and not course.allow_preview()):
            return False

        if lti and course.is_lti() != lti:
            return False

        if lti is False and course.is_lti():
            return not course.lti_send_back_grade()

        return True