Add document score stats to General and Snapshot exports.

[peterdm]

Is your feature request related to a problem? Please describe.
Note: This is about the underlying search engine scores, and not the judgement stats.
In certain situations, search developers may want to establish an engine score "cutoff" where it can help dampen false-positives returned to the client. To analyze whether there is a significant correlation between engine score and relevance, it is useful to return the engine-score from Quepid.

When reporting out cases such as the "General" and "Snapshot" exports, it may be useful to return engine-score aggregates (min, max, mean, std)

Describe the solution you'd like
Produce max(_score), min(_score), mean(_score), std(_score) aggregates upon export.

Describe alternatives you've considered
It's possible to produce this outside of Quepid by specifying the _score field in the Settings tab and producing a "Detailed" export.

Additional context