Making stuff a little bit more concrete

[odarbelaeze]

Added concrete fields to the article class, extra known info about an article can be found in the extra dict, and also the sources that create the article can be found in sources.

Added two named constructors to the Article class, one would build an article based on a whole ISI record and the other would create it based on a reference.

Now citation_pairs returns Tuple[Article, Article], notice that some of the fields in Article might be None

Now CollectionLazy is totally lazy that means it wouldn't return unique articles and such, there will be a Collection class that deals with unique articles and would hydrate the reference article in citation_pairs.

[dsvalenciah]

@odarbelaeze what about add more tests for all new classes?