BibRank: get_self_cited_by_list() input_limit

* Introduces support for input_limit variable in
  bibrank_selfcites_searcher.get_self_cited_by_list().

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reported-by: Ioanna Tzoan Rompou <ioanna.rompou@cern.ch>