zxipcf: partial covering absorption by partially ionized material

This model uses a grid of XSTAR photionized absorption models (calculated assuming a microturbulent velocity of 200km/s) for the absorption, then assumes that this only covers some fraction f of the source, while the remaining (1-f) of the spectrum is seen directly. This is the model used by Reeves et al. (2008) 'On why the iron K-shell absorption in AGN is not the signature of the local warm-hot intergalactic medium', and may also be more generally applicable to the spectral complexity seen in Narrow Line Seyfert 1 AGN (Miller et al. 2007).

par1	column density ($10^{22}$ cm$^{-2}$)
par2	log(xi) where xi=$L/nr^2$
par3	covering fraction
par4	redshift