Remove warnings from read_binary_radolan_file()

[hsonne]

Try to remove these warnings:

url <- "ftp://opendata.dwd.de/climate_environment/CDC/grids_germany/hourly/radolan/historical/bin/2020/RW202006.tar.gz"
files <- kwb.dwd:::download_and_extract_hourly_radolan_historical_bin(url)
grid <- kwb.dwd:::read_binary_radolan_file(files[1L])

Warning messages:
  1: In showSRID(uprojargs, format = "PROJ", multiline = "NO", prefer_proj = prefer_proj) :
  Discarded ellps unknown in Proj4 definition: +proj=stere +lat_0=90 +lon_0=10 +k=0.93301270189 +x_0=0 +y_0=0 +R=6370040 +units=km +no_defs +type=crs
2: In showSRID(uprojargs, format = "PROJ", multiline = "NO", prefer_proj = prefer_proj) :
  Discarded datum unknown in Proj4 definition