get dg_regions from data.genesapi.org

[dpprdan]

This retrieves the dg_regions directly from data.genesapi.org (i.e. https://data.genesapi.org/regionalstatistik/names.json). This ought to fix #18. It is related to #24.

In the long term, i.e. once genesapi supports bildungsdatenbank and ldbnrw, it might be better/necessary to retrieve this data (along with the schema) with a dg_config() (or similar) function at the start of the R session, but until then this ought to suffice.