Increase test coverage

[ocefpaf]

Closes #35

[ocefpaf]

@FloraSauerbronn, as a change of pace and in order to give you a different experience in GSoC. I would like you to review this PR. The review process usually consists of:

1. Check if the tests are passing;
2. Check if the context of the new code is aligned with the PR title and commit messages;
3. Try to understand the code changes, see if they make sense, and try to spot any places for improvements or errors.

You may find that step 3 can be quite demanding but please ask questions, in the PR using the code comment feature of GitHub, to ask for clarifications.