pwab: power-law distribution of neutral absorbers

An extension of partial covering fraction absorption into a power-law distribution of covering fraction as a function of column density, built from the wabs code. See Done & Magdziarz 1998 (MNRAS 298, 737) for details.

par1	minimum equivalent hydrogen column (in units of $10^{22}$ atoms cm$^{-2}$)
par2	maximum equivalent hydrogen column (in units of $10^{22}$ atoms cm$^{-2}$)
par3	power law index for covering fraction