Jupyter notebook: Use pgf code instead of pdf/svg image for export to latex

[Tamas_Papp]

PGFPlotsX should work fine in Jupyter notebooks, and you can save both pdf and svg images, or LaTeX source (pgfplots) directly. See the manual:

https://kristofferc.github.io/PGFPlotsX.jl/dev/man/save.html