Lucene ranking functions

[diegoceccarelli]

Hi *, this is a really interesting project, thanks for sharing :)

I was wondering why for lucene there is only bm25 and not other ranking functions (e.g., lucene's tfidf similarity, kl-divercency etc) that are in the default distribution. Could be possible to add at least lucene's tfidf? recently bm25 became the new default and it would be nice to have experiments that prove that it is better. Thanks