add doctest

[saadmanrafat]

Use the doctest module to run code examples on docstrings interactively. If you run into issues while cloning or creating pull requests? Github's Projected Branches have something to do with it. Our main branch is protected too, but we have to customize the settings. You can easily clone & make sure to create a new branch, i.e. enhancement/add-doctest submit a PR using that branch!