Query: added benchmark test for Cats.

This shows that sorting all pixels in a 5000x5000 tile into
categories, histograms and statistics takes about 1 second.
Query: Made whole tile categories test more realistic.

Now sorts data into 1000 categories instead of 10. This takes
1.35 seconds per tile, an increase of 30%.

Note that this test gererates its own data, so time spent in IO is
not measured.