Calculates angles between PMT combinations according to positions in
    detector_file

    Parameters
    ----------
    detector_file: file from which to read the PMT positions (.detx)
    combs: pmt combinations

    Returns
    -------
    angles: numpy array of angles between all PMT combinations

def calculate_angles(detector, combs):
    """Calculates angles between PMT combinations according to positions in
    detector_file

    Parameters
    ----------
    detector_file: file from which to read the PMT positions (.detx)
    combs: pmt combinations

    Returns
    -------
    angles: numpy array of angles between all PMT combinations

    """
    angles = []
    pmt_angles = detector.pmt_angles
    for first, second in combs:
        angles.append(
            kp.math.angle_between(
                np.array(pmt_angles[first]), np.array(pmt_angles[second])
            )
        )
    return np.array(angles)