Simplify Outposts #2176

This is a functional simplification. This is NOT the exact version that was tested. Beyond the testing, an assignment was removed and a piece changes for consistency.

Instead of rewarding ANY square past an opponent pawn as an "outpost," only use squares that are protected by our pawn. I believe this is more consistent with what the chess world calls an "outpost."

STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 23540 W: 5387 L: 5269 D: 12884
http://tests.stockfishchess.org/tests/view/5cf51e6d0ebc5925cf08b823

LTC
LLR: 2.94 (-2.94,2.94) [-3.00,1.00]
Total: 53085 W: 9271 L: 9204 D: 34610
http://tests.stockfishchess.org/tests/view/5cf5279e0ebc5925cf08b992

bench 3424592

src/evaluate.cpp

@@ -141,7 +141,7 @@ namespace {
   constexpr Score KnightOnQueen      = S( 16, 12);
   constexpr Score LongDiagonalBishop = S( 45,  0);
   constexpr Score MinorBehindPawn    = S( 18,  3);
-  constexpr Score Outpost            = S(  9,  3);
+  constexpr Score Outpost            = S( 36, 12);
   constexpr Score PawnlessFlank      = S( 17, 95);
   constexpr Score RestrictedPiece    = S(  7,  7);
   constexpr Score RookOnPawn         = S( 10, 32);
@@ -305,14 +305,12 @@ namespace {
         if (Pt == BISHOP || Pt == KNIGHT)
         {
             // Bonus if piece is on an outpost square or can reach one
-            bb = OutpostRanks & ~pe->pawn_attacks_span(Them);
+            bb = OutpostRanks & attackedBy[Us][PAWN] & ~pe->pawn_attacks_span(Them);
             if (bb & s)
-                score += Outpost * (Pt == KNIGHT ? 4 : 2)
-                                 * ((attackedBy[Us][PAWN] & s) ? 2 : 1);
+                score += Outpost * (Pt == KNIGHT ? 2 : 1);
 
-            else if (bb &= b & ~pos.pieces(Us))
-                score += Outpost * (Pt == KNIGHT ? 2 : 1)
-                                 * ((attackedBy[Us][PAWN] & bb) ? 2 : 1);
+            else if (bb & b & ~pos.pieces(Us))
+                score += Outpost / (Pt == KNIGHT ? 1 : 2);
 
             // Knight and Bishop bonus for being right behind a pawn
             if (shift<Down>(pos.pieces(PAWN)) & s)