Support sub-volume averaging

In intrange, when the parameter avexy is set to True, the current stack is averaged along "z" axis and the resulting image undergoes the AI. So, we end up with one curve instead of N_z.

aveim = np.average(fil[self.arraypath][()], axis=0)
xye = self.ai.integrate1d_legacy(
    aveim, self.args.npt, unit=self.args.unit,
    polarization_factor=1.0,
    correctSolidAngle=False,
    azimuth_range=self.azims[w],
    error_model=self.args.error,
    method=self.aimethod
)