Improve paper matching function

[barney-walker]

At the moment, if two papers have title 'Unavailable' they will be interpreted as the same paper.

[barney-walker]

this has been improved slighty to Author + Year match but could still do with some work

[pesho-ivanov]

What is 'Unavailable' supposed to mean for a paper?

In the Recommended Reading list I see:

1. All the cited-by papers (which are also visualized)
2. Some other papers -- is there a clear definition on what those are supposed to be?
3. Other papers named 'undefined' -- are these the same as the mentioned 'Unavailable' ones, when does this problem arise and how should it be fixed?