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zigm: UV/Optical attenuation by the intergalactic medium

This multiplicative model computes the mean attenuation of the optical/UV
spectrum of an object at redshift z at a random position on the sky due to
intergalactic medium (IGM) clouds following either
Madau (1995, ApJ 441, 18)
or Meiksin (2006, MNRAS, 365, 807).
The model calculates the mean expected attenuation due to resonant scattering
by Lyman transitions and photoelectric absorption shortward of the Lyman limit.
Attenuation by Helium and metals are not included in the Meiksin model, but
are expected to be small. The total attenuation is set to zero for wavelengths
less than 900 Angstroms. The user chooses whether to include attenuation due
to photoelectric absorption.

par1 |
z: redshift |

par2 |
model: use Madau(0) or Meiksin(1) |

par3 |
include photoelectric absorption(1) or not(0) |