[WIP] improve/shorten the mates in fishmates.epd.gz

[robertnurnberg]

New stats:

> python matestats.py fishmates.epd.gz && mv fishmates.png images/
Loaded 1000000 unique EPDs with |bm| in [3, 35].
Average for |bm| is 10.19

Number of improved mates and/or PVs:

> python diffmates.py fishmates.epd.gz ../matetools.master/fishmates.epd.gz | grep '>' | wc -l
30423

Now includes perfect mate scores and PVs for all puzzle positions that have 6 pieces or fewer. Thanks to new Lichess 6men DTM tables.