Consistent naming

[Thomas-Lemoine]

The difference between an item, an entry and an article is often unclear or mixed, and it could be clarified within the method and variable names as well as some type signatures. Also, fetch_entries and make_data_entry and unprocessed_items and process_entry would benefit from clearer naming schemes (for example, process_entry could return the data_dict instead of self.make_data_entry(data_dict), which makes an article so should be renamed to make_article, etc.) Not really critical, mostly just helps avoid bugs down the line. There's also two AlignmentDataset classes, one of which would make more sense to be named a SourceHandler which all different source handlers inherit from, potentially? Not sure though