Add unit-tests

[damianooldoni]

Continuous integration is powerful. It would be even more powerful if we have tests for our functions. I try to do it immediately while writing the function itself or immediately after before starting a PR. In this way, we are more confident nothing breaks. I don't want to force all of us to write tests for the past as it will take a lot of time. My proposal is the following;
Do you write a new function or do you modify an existing one? Add unit-tests to your to-do-checklist before writing a PR.

There are some tests already in the ./tests/testthat/ directory you can use as examples.