Rewrite construct_df and add tests

[gedeck]

Closes #16, #14, #9

• Function construct_df is rewritten to work with the most recent pandas version.
• Add infrastructure to run tests in docker containers
• Add github actions to run tests using different Python versions (only done if github actions is enabled for this repository, free for public repositories)

See e.g. https://github.com/gedeck/doepy/actions/runs/3165946032 how github actions work

[hoechenberger]

Hello! What's the status on this? :)

[gedeck]

I don't know. I use the package in an upcoming book (https://gedeck.github.io/mistat-code-solutions/IndustrialStatistics/) and was hoping that it would get fixed soon. Unfortunately, this hasn't happened yet, so I incorporated the code into the package that accompanies the book (https://github.com/gedeck/mistat, https://pypi.org/project/mistat/; import using from mistat.design import doe).

I'm not happy with this solution but it was the best I could do for now.

[hoechenberger]

Thanks for your swift response, @gedeck!!