Fix headings on type system slides
On strings, `in` is not a membership test
No duplication in sets
Small change in example loop
Add exercises for matplotlib
Add all authors to all lessons
more git and gitlab
Merge branch 'master' of hg-git01.lumcnet.prod.intern:humgen/programming-course
fixing some sub-slides
update Outs for git notebook
Add xkcd example and slides on backends
Merge remote-tracking branch 'github/master'
Add welcome slides