API: Implement individual file downloading through the API

[pdurbin]

In various ways, users have been asking for how individual files can be downloaded via the API. @rliebz opened a Trello card about this and various support tickets have been opened, most recently RT #192990. There's also a thread on the Google Group called "Direct access to data via URL?". @leeper chimed in to express interest in this feature.

This ticket is about downloading individual files via the API.

(There's a related ticket at #183 that's about downloading all the files from a dataset via SWORD, probably as a zip file.)

As far as I understand, downloading individual files has been more or less implemented already but it hasn't yet been documented at http://guides.dataverse.org/en/latest/api , so users are understandably confused. For example, I wanted to expose the URLs for individual files via the Search API in #69 (and did) but I played around with the GUI to try to figure out what the URLs should be to download individual files.

Since @landreev implemented file downloading, I'm going to assign this ticket to him. I'm vaguely aware that the API might support fetching different versions of files (original vs. CSV) and from what I understand API tokens can be used to download restricted files (this is described at #1228 (comment) ). (My little proof-of-concept Dataverse Android app is able to download unrestricted files just fine without a token, as expected.) I believe all that remains is to document the behavior so we can send API users a link to the appropriate section of the API Guide.

[landreev]

This was given to me just a documentation task, to add something describing our Data Access API to the guides. I added a section on it to the "API Guide".

[kcondon]

OK, documentation is present. I believe the basic download singular and bundle functionality was tested in another ticket. The rest of the API needs to be tested but closing this ticket as API documented.