Fixed nan returned for ndcg, rprec if no relevant docs retrieved

[TSoli]

I noticed that if no relevant documents are retrieved for a run then NDCG, Rprec and recip_rank all returned nan. For NDCG and Rprec I believe these should be 0 instead. I left recip_rank since it is not obvious which would be preferred here.

I wrote some tests for the fixes I made which ensure that the relevant metrics are 0 if no relevant documents were retrieved or if the documents retrieved were all unlabelled.

Note this is different from #25 since there are relevant documents they were just not retrieved during the run (or up to the depth evaluated). With regards to #25 it seems that the entries for NDCG per query do not exist if there are not relevant documents in the collection. This is not fixed here.