More accurate pawn attack span definition
Tweak the pawn attack span for backward pawns and the zone behind
opponent opposing pawns. This is important in positional play and
one of weaknesses of the engine in recent high level games.
STC
LLR: -2.95 (-2.94,2.94) [0.50,4.50]
Total: 66843 W: 14884 L: 14717 D: 37242
http://tests.stockfishchess.org/tests/view/5d8dcb1b0ebc590f3beb2956
LTC
LLR: 2.96 (-2.94,2.94) [0.00,3.50]
Total: 77699 W: 12993 L: 12602 D: 52104
http://tests.stockfishchess.org/tests/view/5d8de9bc0ebc590f3beb3d00
See discussion in https://github.com/official-stockfish/Stockfish/pull/2332
Bench: 4012371