A new tutorial for `Field.mesh()`

[LSchueler]

We should add a tutorial showing how to use the Field.mesh method to generate random fields on pre-existing meshes.

[MuellerSeb]

You are totally right! I'll take a look at this next week.

[banesullivan]

It'd be awesome to have this tutorial use PyVista 😉

[LSchueler]

Why not provide 2 or more tutorials? - Maybe you already have something like a minimal working example?
I can also beef it up a little with text, if you just want to provide the mesh file and the code.

[MuellerSeb]

There are now some tutorials with MeshIO: #112. PyVista examples still missing, but the interface is now ready: #59

[MuellerSeb]

ogs5py has its own tutorial: https://geostat-framework.readthedocs.io/projects/ogs5py/en/stable/tutorial_02_pump.html