Implements the order dependent family of functions defined in equations
        4 to 7 in the reference paper.

def _J(self, theta):
        """
        Implements the order dependent family of functions defined in equations
        4 to 7 in the reference paper.
        """
        if self.order == 0:
            return np.pi - theta
        elif self.order == 1:
            return tf.sin(theta) + (np.pi - theta) * tf.cos(theta)
        elif self.order == 2:
            return 3. * tf.sin(theta) * tf.cos(theta) + \
                   (np.pi - theta) * (1. + 2. * tf.cos(theta) ** 2)