Show Matern kernel parameter effects in demo 4

To understand the add_fluctuations_matern() parameter effects better, I added showcases of them to the 4th demo (getting_started_4_CorrelatedFields.ipynb).

@gedenhof: could you have a look at the parameter explanations and check whether they are correct, please?

Also: I played around with the values for some time, but if you know better/more practice-oriented value combinations to showcase, please amend them.

Cheers, Lukas