PDF plotting should be in a separate file

Created by: Zaharid

The PDF plots are a large enough amount of code and stylistically different enough that they should probably be in their own file, along with some documentation on how to extend them (which is easy if you know what to do modulo some warts I have to fix).

Besides the sheer size of plots.py bothers me already.

There are several questions on how to proceed.

• Should we move the classes and keep the plot definitions (I think not). Or should we put the classes in yet another file (I also think not).
• Should we move things like the effective exponents to this new file (maybe not)?

@nhartland any opinion since you were wanting to change this?