Flip the direction.

    Parameters
    ==========
    zenith, azimuth: neutrino origin
    radian: bool [default=True]
        receive + return angles in radian? (if false, use degree)

def source_to_neutrino_direction(azimuth, zenith, radian=True):
    """Flip the direction.

    Parameters
    ==========
    zenith, azimuth: neutrino origin
    radian: bool [default=True]
        receive + return angles in radian? (if false, use degree)
    """
    azimuth = np.atleast_1d(azimuth).copy()
    zenith = np.atleast_1d(zenith).copy()
    if not radian:
        azimuth *= np.pi / 180
        zenith *= np.pi / 180
    phi = (azimuth - np.pi) % (2 * np.pi)
    theta = np.pi - zenith
    if not radian:
        phi *= 180 / np.pi
        theta *= 180 / np.pi
    return phi, theta