Add the ability to list a set of arc line files

This will allow me to use dome light (== Hg + Ar) + sky (OI, Na) to solve for SuNSS.  

We should also split the monolithic ArCdHgKrNeXe.txt file but that's a different ticket (e.g. we need ArHg.txt separately from Ar.txt as the blends are different, and the broadened lines that we don't want to use are only in the former)