Code documentation

This branch adds bunch of comments to the source code, mostly comments about functions. I went through all the .py files in 'src' folder, except api.py since that was still under construction. As I only added comments, no functionalities were changed except the division-by-zero sentry log test, which I commented out. There may be small inaccuracies in the comments, as I also commented code I hadn't personally written, so if you find some mistakes let me know or just fix them yourself.

I also added more specific documentation in weekly report about the bugs I fixed, as that apparently was something that was asked in week 5.