add doc tests

[isaacsas]

We have lots of code examples throughout the docs now, so it would be good to set up tests to make sure they actually keep running and working rather than having to manually test them by hand.

[TorkelE]

Seems we've been burned on this a few times already now...

Is there an automatic-ish way of doing this, or would one just copy the code that's in the docs into a test file (and whenever stuff are added/changed in docs change this)?

[isaacsas]

I think there is a way to label documentation strings as a doctest and then when the docs are being built it tries running the code.

[TorkelE]

I have started working on this, but there are some issues over at https://github.com/JuliaDocs/Documenter.jl/issues which probably need to be sorted out before it would be possible to make sensible doctests. Hopefully, it won't be too long.

[TorkelE]

Given that we changed our doc approach, is this superseded?

[isaacsas]

I think it is still worth keeping this open, this would more be about testing the docstrings, which I don't think we can really do at the moment the way we'd want.