Implement Dataset add_item
Thanks @lhoestq for your remarks. Yes, I agree there are still many issues to be tackled... This PR is just a starting point, so that we can discuss how Dataset should be generalized. Sure ! I opened an issue #1877 so we can discuss this specific aspect :) I am going to implement this consolidation step in #2151. Sounds good ! I retake this PR once the consolidation step is already implemented by #2151.
Implement `Dataset.add_item`.

Close #1854.