forked from: radian角度をπ範囲に丸める

@see http://tail.s288.xrea.com/open/index.php?%A5%E1%A5%E2%BD%F1%A4%AD%2Fradian%B3%D1%C5%D9%A4%F2%A6%D0%C8%CF%B0%CF%A4%CB%B4%DD%A4%E1%A4%EB