[Documentation] Updating yt usage in docs.

Hi Tom,

I'm at the HIPACC summer school here at UCSC and Stella Offner just finished telling us about your code (first I've heard of it, I'm ashamed to say) and it's awesome integration with yt.

I looked over the docs and noticed that you're using the old, soon-to-be deprecated plotting routines.  This change updates the example script to use the new ProjectionPlot routine, which is more full featured and somewhat easier to approach.

Unfortunately I couldn't update the images but I think that would be easier to do on your end.  You'll also be able to spot if I made a mistake in the updated script.

Very much looking forward to trying Hyperion out in the near future.  Recently Andrew Myers wrote a RadMC3D exporter and added it to yt so all supported codes can dump inputs for RadMC3D - hopefully we'll be able to do something similar for Hyperion.