Quantities

I have made a number of changes to section 2. Everything was basically correct before, but I was a little confused by the structure. It didn't help that Rohlfs and Wilson (at least in older editions) has an error in the section where they lay out the N(total) CO column density equation. 

Hopefully it is still/more clear what's going on. The most important changes I made to the code were to change the units on the synthetic code to just be K and to wait to sum over the spatial pixels until the line where we actually calculate the cloud mass (i.e. keeping things in column density for longer). I also expanded the discussion with some comments about the assumptions involved here and some snide remarks about radio astronomers and their annoying units/conventions.