Support matplotlib

[patscott]

It would be nice if pippi could do everything with either a ctioga2 backend or natively in Python via matplotlib.

Probably the modular way to achieve the latter is to have pippi script write a python script that can be run with pippi plot to generate the pdfs with matplotlib. This would happen either instead of, or in addition to, generating the bash script that calls ctioga2, which pippi script currently generates.