Merge pawn count terms using their average
This simplification patch merges the pawn count terms in the eval
formula with the material term, updating the offset constant for
the nnue part of the formula from 34000 to 34300 because the average
pawn count in middlegame positions evaluated during search is around 8.
STC:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 138240 W: 35834 L: 35723 D: 66683
Ptnml(0-2): 527, 16587, 34817, 16626, 563
https://tests.stockfishchess.org/tests/view/6653f474a86388d5e27daaac
LTC:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 454272 W: 114787 L: 115012 D: 224473
Ptnml(0-2): 246, 51168, 124553, 50903, 266
https://tests.stockfishchess.org/tests/view/6654f256a86388d5e27db131
closes https://github.com/official-stockfish/Stockfish/pull/5303
Bench: 1279635