Rework the menu

README.md

_config.yml

_includes/head.html

_includes/header.html

_includes/menu_single.html

_includes/menu_sub.html

_includes/posts.html

_includes/print_number.liquid

_layouts/default.html

news/posts.html

people/mkohlhase-activities.md

people/mkohlhase-cv.md

people/mkohlhase.md

people/twiesing.md

projects/current.md

projects/former.md

public/materialize/css/materialize.css

research/formal-methods.md

research/kminteract.md

research/overview.md