A note on the gabs model

Oleg Kargaltsev has pointed out that the gaussian absorption line (gabs) is badly documented. The third parameter is called Tau although it is not the optical depth. The on-line help page has been amended and the gabs entry in model.dat will be changed so that the third parameter is just called Strength. In practice, if the optical depth is small then the third parameter is the equivalent width of the line.