Option to run doctests

[hombit]

Currently pytest looks into tests and runs tests from there. It would be also very convenient to run docstring examples as docstrings with CI and pre-commit. See pytest doctest docs here:
https://docs.pytest.org/en/4.6.x/doctest.html

[hombit]

I believe #381 is related to this one

[delucchi-cmu]

Are you suggesting we add to the default test targets in the pyproject toml? Or just add some documentation in the pytest page about how you could set it up?

[hombit]

@delucchi-cmu I suggest adding the default test target