Remove static image files whenever possible

[emmanuelle]

A majority of the image files of the repository could be generated from Python scripts. Use

find . -name "*.jpg"
find . -name "*.png"

to find them.

[emmanuelle]

However, some of these images are in chapters that need to be rewritten (like the scikit-learn chapter, see #245), so first go through the list of open issues before working on generating the images.

[gertingold]

I could contribute some of the schematic figures, e.g. in the introduction, but these would be programmed using PyX. The package is pip-installable, but I am not sure whether it would be acceptable as a dependency for the SciPy lecture notes.

[gertingold]

A first step is made in #392 in order to address #372. The image is in a sense still static but now a source is available, thus allowing for improvements of the image.

[stefanv]

Rendering from SVG is another option, that is employed e.g. for the cover already.