Adapt repo to r-package structure

Adapt all necessary steps to change the repo structure to that of an r package as outlined in https://r-pkgs.org/.

• R source file in ./R
• call functions with respective namespace, no library or require statements
• update examples
• move data for examples in ./data/
• set-up packrat for dependency managemnet
• add license
• add respective packge dev files (NAMESPACES, DESCRIPTION)
• document all functions using roxygen2
• implement tests
• ~configure pipline for automated testing~~
• add Readme.Rmd, Readme.md
• check() without warnings
• install() without errors