Returns a StudentGrades model for the regid and term.

def get_grades_by_regid_and_term(regid, term):
    """
    Returns a StudentGrades model for the regid and term.
    """
    url = "{}/{},{},{}.json".format(enrollment_res_url_prefix,
                                    term.year,
                                    term.quarter,
                                    regid)
    return _json_to_grades(get_resource(url), regid, term)