Neutrino direction in polar coordinates.

    Downgoing event: theta = 180deg
    Horizont: 90deg
    Upgoing: theta = 0

    Angles in radians.

def theta(v):
    """Neutrino direction in polar coordinates.

    Downgoing event: theta = 180deg
    Horizont: 90deg
    Upgoing: theta = 0

    Angles in radians.
    """
    v = np.atleast_2d(v)
    dir_z = v[:, 2]
    return theta_separg(dir_z)