def to_euler(self):
        '''find Euler angles (321 convention) for the matrix'''
        if self.c.x >= 1.0:
            pitch = pi
        elif self.c.x <= -1.0:
            pitch = -pi
        else:
            pitch = -asin(self.c.x)
        roll = atan2(self.c.y, self.c.z)
        yaw  = atan2(self.b.x, self.a.x)
        return (roll, pitch, yaw)
find Euler angles (321 convention) for the matrix