Mapping from county to RegioStaR category

[annawendler]

RegioStaR categories (https://bmdv.bund.de/SharedDocs/DE/Artikel/G/regionalstatistische-raumtypologie.html) describe different types of regions, considering if they are shaped more urban or more rural. We want to have a mapping from the county-level that we are already using to their respective RegioStaR types where we are mostly interested in RegioStaR7 and RegioStaR4.

This dataset could be used for this:
https://bmdv.bund.de/SharedDocs/DE/Anlage/G/regiostar-referenzdateien.xlsx

These categories are interesting for our regression model for the analysis of NPIs, see #864.