Add Pubmed (citation + abstract) dataset (2020).
LGTM! ftp addition looks fine but maybe have a look @thomwolf ? It's not finished yet, I need to run the tests on the full dataset (it was running this weekend, there is an error somewhere deep)
 @yjernite Ready for review !
@thomwolf 

So I tried to follow closely the original format that means I still had to drop information (namely tags on elements are simply discarded for now but they don't seem to carry critical information).
Some elements are also discarded they tend to not come up often:
 - The most notable is Author affiliation, which seems to be all over the place in terms of what it look meaning it's hard to actually get a consistent format.
 - Journal is the same, all the elements in there can be wildly different so I decided to drop it for now instead of trying to figure out a way to have a common presentation. (the DOI and medline ID are present so it can be recovered).

I think this PR could go as it but we probably should add a way to get easier information to use with a config.
For instance `{"title": "string", "abstract": "string", "authors": List[str], "substances": List[str]}` maybe ? (substances for instance is a tricky one, some substances only have an international identifier, some have simply a common name, some both)

It's relatively easy to do I think it's mostly discarding other fields and renaming some deep structure into a flat one. Look ok to me but @lhoestq is the master on the Download Manager side