Add example code for common use-case of generating some simple plot

[apirogov]

This was actually a suggestion we got at a poster presentation of the template and sounds like a useful idea.

First a good choice of library must be done (matplotlib vs bokeh vs plotly vs dash vs panel) - simple enough, but "good practice". Next, an example should be built that demonstrates the plot generation

• invoked from a simple CLI
• or done interactively in Jupyter

or even both ways. This needs some further thought from the point of view of "didactics" - what and how much to show.

Should wait with this in any case until #16 is done.