[Documentation] Include example of how to use results

[sjvrijn]

Related: openjournals/joss-reviews#2974 and #23

The Solution-class used in this framework as a default output is not so intuitive/standard that it can be included unexplained.
A (simple) example of how to extract and use each type of returned/stored data for typical uses would be very helpful, so something like

• plotting a typical 2D pareto-front
• obtaining the particular solution for some f (to use your nomenclature)
• using the included distance attribute

Using https://documentation.divio.com/ terminology, ideally you would also include some explanation in a 'tutorial', but I think that's more optional than adding the list above as a set of 'how-to's.

[geoffreyp]

I add a section in each example and a new tutorial to extract and plot the Pareto Set and Pareto front. PR #53