Copy plotting back from Python package

[felixhekhorn]

Actually, after a bit more thinking I believe we can do even more, i.e. almost what I want:

1. I suggest to split all the static stuff, i.e. the plot functions, away from pineappl_cli/src/plot.py into a file pineappl_py/something/plot.py
2. that new file should be statically included into the pineappl Python package (and thus the Python interface will have easy access to the plot functions; we can also provide some additional methods which generate the necessary data from a grid using the Python interface itself)
3. the CLI will copy that file verbatim into the output of pineappl plot

Originally posted by @felixhekhorn in #312 (comment)