added un_ga dataset
Looks like this PR includes changes about many other files than the ones for un_ga

Can you create another branch an another PR please ? @lhoestq, Thank you for suggestions. I have made the changes and raised the new PR https://github.com/huggingface/datasets/pull/1569. 
Hi :hugs:, This is a PR for [United nations general assembly resolutions: A six-language parallel corpus](http://opus.nlpl.eu/UN.php) dataset