zbabs: EUV ISM attenuation

The ISM attenuation due to neutral H, neutral He and once ionized He. This is a modified version of the Rumph et al. (1994) model, using the ismatten program by Pat Jelinsky, and allows the user to set the model redshift.

par1=$\eta_H$ H column density ($10^{22}$ atoms/cm$^2$)
par2=$\eta_{HeI}$ HeI column density ($10^{22}$ atoms/cm$^2$)
par3=$\eta_{HeII}$ HeII column density ($10^{22}$ atoms/cm$^2$)
par4=z redshift