Version 0.21.0

• Remove duplicate documents when collecting Vespa features with app.collect_training_data_point (#345)
• Create app.collect_vespa_features and app.store_vespa_features that use app.query_batch to speedup Vespa feature collection. (#347)
• app.collect_training_data_point and app.collect_training_data will be deprecated in future releases and should be replaced by app.collect_vespa_features and app.store_vespa_features.