Add an example of unit test

The following discussion from !3 (merged) should be addressed:

• @ijakovlj commented on a discussion: (+5 comments)

  I would love to see how you do unit tests at CERN. This has always been a hard thing to do for python projects.

  Regarding this structure, this is more for creating Python Modules, that can me shared via PyPi. I mean great things are mentioned here, definitely a good read. I have always used the The Hitchhiker's Guide to Structuring Your Project for project structure. Maybe you could have a look at a existing CERN project and compare the structure, then we can figure out how they differ and if it makes sense for us to integrate things. Because the pypip and pre-commit are amazing tools.