Make unit tests

[balintlaczko]

This is a long-term goal, in order to keep the growing package smooth and bug-free. Currently we mostly test by running the (by now, quite long) jupyper notebook, which is better than nothing, but as the last few issues have shown, there can be bugs which only happen outside the notebook event loop/environment, which will get completely missed by this method. Ideally, we need to create unit tests that test both inside and outside a notebook-like event loop, and can be scheduled to run after every single new commit. It will take some time to write this library of tests, but we have to start somewhere... :)