Release v0.2.0

• add contributors Michael Rustler, Lukas Guericke
• use GitHub actions instead of Travis and Appveyor
• add tests
• update dataset "dwd_files""
• modify functions
  • get_radolan_url(): add arg "ftp_root"
  • get_radolan_urls(): add args "..."
  • list_url():
    • remove args "depth", "curl", add arg "full_names"
    • reimplement using kwb.utils::listToDepth()
• add new functions
  • calculate_masked_grid_stats()
  • check_or_download_shapes_germany()
  • get_berlin_dwd_mask()
  • get_example_grid_germany()
  • get_shapes_of_germany()
  • load_potential_evaporation_berlin()
  • load_potential_evaporation_berlin_2()
  • load_precipitation_berlin()
  • open_description()
  • read_asc_gz_file()
  • read_daily_data_over_shape()
  • unzip_asc_gz_file()
• export functions that were private before
  • coordinates_to_EPSG_4326()
  • extract_radolan_zip_files()
  • get_radolan_extension()
  • get_radolan_projection_string()
  • radolan_raw_to_raster()
  • write_raster_to_file()