Get doctests to pass

[jagerber48]

During #279 I discovered that there are doctests in the documentation, however, many are failing. On my tests run 111/204 doctests failed. We should get these doctests to pass. It would also be a good idea to run doctests to run in the CI.

[andrewgsavage]

this would prevent issues like #286 yea

[newville]

@jagerber48 @andrewgsavage Sorry to be dense, but I don't understand. Are there doctests in the documentation? Where are these? I don't see a single doctest or testcode directive in the rst files.
If there are such tests, how do you run them? I have no idea.

I've always found code in docstrings unreliable for testing, as they may be incomplete snippets. Do we want to run these and count "passing" and "failing"? I could be persuaded, especially if this was included in the CI, but it might be real effort...