Consider adding CI tests

[larsvilhuber]

While the package hasn't been updated in a while, it surely may be in the future. You might consider running some automated in-the-cloud tests.

• see this workflow for an example on how to do that
• you can post IPUMS extracts if they accompany a paper, which in this case is fine. Of course, you might want to explicitly subsample the data anyway to allow for smaller/faster tests for illustration only.