Add unit tests for the `cli` module! I have not figured out yet how to test the CLI functions properly with `pytest`. [Existing tests](https://github.com/volkamerlab/kissim/blob/6ec2e93f1b0d6a8426cbb156b5ca127d3f1fc62b/kissim/tests/cli/test_encode_compare.py#L30) caused problems with `codecov` and I do not understand why, so I am not trusting them for now, see PR https://github.com/volkamerlab/kissim/pull/33.