Merge pull request #204 from ngeiswei/fix_precision_bscore_best_score

Add precision bscore best score unit test
Add assert to check for negative distance + log in diversity
Fix bug in max candidate merging
Fix bug in precision_bscore scoring log
Add comment about Tanimoto being possibly negative + remove useless space
Merge branch 'master' into origin-fix_precision_bscore_best_score