[R] the R code needs to be split

[JonasCir]

Title say's it all :)
Split the sormas-oegd-credible-testdata.R into at least the following files:

1. a file/function which creates the parameters from the RKI dashboard/page and stores them on disc. By this we don't need to run this step every time
2. The computation of the entities. In the best case. split the generation of the different entities in a single function each (e.g., generate_case, generate_contact etc)
3. a file/function which does the storing of the previous data
4. A function (i.e., main) which calls all the previous functions

[stephaneghozzi]

a file/function which creates the parameters from the RKI dashboard/page and stores them on disc. By this we don't need to run this step every time

At the moment there are two data sources: RKI's corona dashboard and RKI's SurvStat. The first is already queried automatically, the second however has to be downloaded manually, although there's a way to access them via a web service, see issue #21