Filter empty `clear_text`

Introduce stratified folds

Update `num_to_words`, `filter_and_clean_text` & `preprocess_text` and add `map_special_terms`

Use json instead of jsonl

Rename and reorganize

Add methods `_remove_to_long_pauses` and `_filter_utterances_by_duration`

Add methods to split long chunks and remove trailing pauses

Initial Commit