Plotting: tests, examples

[PMeira]

The plotting tools are marked as experimental, yet almost complete.

This is a separate ticket to #3 to complement the plotting system with some more features, and add the required tests to ensure we don't break it accidentally.

Examples shall also be added when it's stable enough.

[PMeira]

The new example notebook covers most types of plots and a bit more:

https://github.com/dss-extensions/dss_python/blob/master/docs/examples/Plotting.ipynb (without images)

or

https://dss-extensions.org/dss_python/examples/Plotting/ (with images)