v1.3.0 -- Added differential attenuation model

This new version includes a new major feature: differential attenuation. In the original dust model of FAST, all stars suffer the same amount of extinction by dust. With differential attenuation enabled, young stars can be given an extra amount of extinction, since they can be assumed to still reside in their birth clouds (this is the Charlot & Fall 2000 prescription). This feature is disabled by default but can be turned on in the .param file, and the amount of extra extinction in birth clouds can be varied on a grid (or fixed to a single value).

A consequence of this addition is that it is now possible to apply the dust luminosity prior either on the whole galaxy, only on the birth clouds, or only on the cirrus (diffuse ISM) components.