'extinction' incorrectly maps to temperature section of irsa_dust result

We need to change "extinction" to "temperature" in a few places.

There are three sections in the IRSA dust service responses: "E(B-V) Reddening", "100 Micron Emission" and "Dust Temperature". The user can get data on a specific section by passing a section argument in some irsa_dust methods. The allowed values for the section argument are "ebv", "100um" (which make sense) and "extinction" - which should really be "temperature", since it returns the "Dust Temperature" section.

I have a fix for this, but I'll wait until after #244 is accepted before issuing a pull request, in order to keep things simple.