raw phosim magnitude calculator

A framework for converting phosim input file parameters into actual ugrizy AB magnitudes.  Sorry only sparse comments yet!

One question: for computing extinction I stole a file from astropy.specutils.  That repo is BSD licensed, which from my very "I am not a lawyer" point of view looks to me means it's okay to copy said file into this repo, as long as we propagate the BSD license and don't blame the astropy developers for anything.  Maybe someone other than me knows the details of this better?

-Josh