Moving from ARFF to Parquet

[PGijsbers]

Roadmap (when #1029 is merged):

• wait for production server to include minio references in the dataset description (hence serverside)
• update the unit tests when the test server has a separate dedicated minio instance
• do an investigation to see if, or for which datasets, pickle support should be maintained (see #1027)
• remove downloading and processing of arff files by default
• determine if manual download of arff file should still be supported (I'd argue at least in the immediate feature that's a good thing)