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) 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