Code docs, rewrite of app.R for better overview

[Claudius-Appel]

• [ ] - further segment out the callback code into addressable functions
• [ ] move away from the func(input, DATA, DEBUGKEYS)-syntax, instead have each function import the variables it actually needs
  • the reasoning here is that then they behave like normal functions, and thus can also just be tested like normal functions
  • ALT: figure out if I can just supply a input <- list(X = Y)-format as well, or if the "special" behaviour of shiny components INPUT and reactives DATA/DEBUGKEYS is intrinsically required by the app.

[Claudius-Appel]

This will not be imlemented, development of this project is too far gone already.

Additionally, it doesn't really make any sense as otherwhise functions would often have to receive an enourmous number of arguments, which will just make them unreasonably complex and make maintenance pointlessly hard.