Test docstring examples as part of CI runs
Created by: femtobit
This PR adds a step to out GitHub Actions CI runs which executes all example sections in our docstring comments in order to make sure they are up-to-date and working as expected (f221a304).
Unsurprisingly, this has unearthed a lot of failing examples (mostly because of using old API or incomplete imports) which are fixed here as well (10b1baab).
Note that if a docstring example really should not be tested, it can be skipped by adding
>>> import pytest; pytest.skip("reason")
as I've done here (only) for some NETKET_EXPERIMENTAL
code.