CoNLL StreamExtractor always outputs the entire block of prefixes, regardless of which are referenced

[leogott]

Mostly a cosmetic issue, but even this (lone) comment gets the full treatment of eight prefix statements.

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This is probably quite low priority, but something I'd like to track.

[chiarcos]

Post-filtering over the output is trivial, so I haven't felt the need for that, so far. A potential problem is that downstream applications might rely on prefixes for identifying blocks of information. In any case, post-filtering should be optional.