Add running examples to CI

[mastricker]

Some of the examples did not work when I checked them. Since this is probably the first place where people look at our baby when they want to try something out we should ensure that they work.

Putting @rosecers since you seem to be the driving force behind the examples/tutorials and that could be included? Feel free to object.

[Luthaf]

Very good idea :+1:

I would also take the occasion to cleanup the examples, and maybe reduce their number. For example, I think the playground.cc, test_sph_harm_poles.cc and maybe others files should go to code heaven.

[mastricker]

playground is literally what it's name is. But it should not reside in examples. Didn't we have that discussion a while ago that we allow a sandbox - at least for a while to try stuff.

But in general, I agree.