[fix] idf calculation by using pre-calculated documents count

Closes #7 (closed)

Until now there was a big mistake in the idf calculation where N=2 from the exposé draft was hard coded as the total number of documents in

log(N / df)

Now we count all documents by

final DataSet<Long> documentsCount = Count.count(logicalGraph.getVertices());

and broadcast this value which is then used in TFIDF.java

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This effect the previous first observations made in !3 (merged) and also resulting in a significantly better performance.

1. Previous benchmark without documentsCount

2. Benchmark with documentsCount

At least this weird spike from Img 1 is gone now and overall we see a higher trend in Precision, Recall and F-Measure.

But it still remains slightly ambiguous where the best threshold is but this is under further investigation.

added bug label

mentioned in merge request !3 (merged)

mentioned in merge request !4 (merged)

changed the description

mentioned in commit 4df0c075

merged