Prevent the second operation in func_plot.py to overwrite the first one, which
set up the Agg backend.

Add matplotlib.use('Agg') to the code examples, which is necessary when run in
an environment without X11 sever, such as the CI pipeline.

Add a chapter showing and example how to plot a simple function with
matplotlib. This commit add eps-files to the artifacts of the CI.

This commit adds a chapter about the installation of python3, numpy, scipy and
matplotlib and a chapter with a first 'Hello World'-like example.