Fixes:
- EpochRunner: Fixed wrong number of validation examples and result_dict possibly not initialized
- extrapolators: Fix extrapolators sometimes reducing precision
Changes:
- test_extrapolators: Test actual output instead of just running experiments
- Factory: Allow instantiation by class name only
- Treat held-out test sets and validation sets separately
- docs: Added JSONDataset documentation
- test_epoch_runner_load_save: Test all combinations of held-out test and validation sets
- keras_idk_hc: Adapt CHILLAX terminology
- Fixed NLTK version because 3.6.6 renames omw corpus to omw-1.4