Computing F1 score for predicate mentions

https://github.com/vered1986/OKR/blob/master/src/baseline_system/eval_predicate_mention.py#L35

We shouldn't be using average of precision and recall for computing F1 but the harmonic mean.
Can we change/modify the method compute_predicate_mention_agreement in predicate_mention.py: 
https://github.com/vered1986/OKR/blob/master/src/agreement/predicate_mention.py#L31
