F-Measure and similar metrics

[agazzarini]

At the moment the Metric framework allows to implement a metric which is able to compute a value, at query level, after being notified on each search hit. At upper levels,the same metric is aggregated using the arithmetic mean.

Metrics like F-Measure need a different infrastructure, because they are computed on top of other metrics (Precision & Recall for F-Measure)

[agazzarini]

Composite metrics structure have been implemented in the latest commits. For a concrete example you can check the F-Measure metric code. RRE includes the three most popular versions of that metric: F0.5, F1 and F2