Bugfixes and simplifications

[bonartm]

some suggestions and bugfixes

• put code back in the mobydick folder
• used python -m pytest instead of pytest to test modules in another folder
• corrected import statements
• removed classes around test functions
• removed articles that are not up to date anymore
• minor bugfixes

[krother]

I took a closer look at your PR. Thanks for the suggestions! They all make sense to me. The only blocker is that I use an incompatible deployment process. I'd have to branch the mkdocs part out of the PR. Should be possible.

[bonartm]

I will open a new pull request that just contains the content updates/ fixes.