pcfabs, zpcfabs: partial covering fraction absorption

A partial covering fraction absorption. The relative abundances are set by the abund command.

$$M(E) = f \exp[-\eta_H \sigma(E)] + (1-f)$$

where $\sigma(E)$ is the photo-electric cross-section (NOT including Thomson scattering) (see phabs) and:

par1=$\eta_H$	equivalent hydrogen column (in units of $10^{22}$ atoms cm$^{-2}$)
par2=f	dimensionless covering fraction $(0<f\leq 1)$

The redshifted variant zpcfabs is given by:

$$M(E) = f \exp\left[-\eta_H \sigma(E[1+z])\right] + 1-f$$

where $\sigma(E)$ is the photo-electric cross-section (NOT including Thomson scattering) (see phabs). Relative abundances are as for pcfabs. Parameters are:

par1=$\eta_H$	equivalent hydrogen column (in units of $10^{22}$ atoms cm$^{-2}$)
par2=f	dimensionless covering fraction $(0<f\leq 1)$
par3=z	redshift