Adding Europarl Bilingual dataset
Implementation of Europarl bilingual dataset from described [here](https://opus.nlpl.eu/Europarl.php).

This dataset allows to use every language pair detailed in the original dataset. The loading script manages also the small errors contained in the original dataset (in very rare cases (1 over 10M) there are some keys that references to inexistent sentences).
I chose to follow the the style of a similar dataset available in this repository: `multi_para_crawl`.
