Write unit tests

[dorianim]

As I learned from Prof. Pretschner (my favorite prof so far:), every code should have unit tests.
I am not sure if we can test everything over here, but at least some tests should be possible.

• Write tests
• Setup CI with codecov.io

The tests should run on every commit. Merging should only be allowed if all tests are passing and the added code is covered by tests.

Progress can be seen here:

[dorianim]

Codecov needs to be setup on prs

[dorianim]

Seems to work: #46 (comment)