Use a formula for chain membership bonus

Passed both short TC:
LLR: 2.96 (-2.94,2.94) [-1.50,4.50]
Total: 5087 W: 1072 L: 951 D: 3064

And long TC:
LLR: 2.95 (-2.94,2.94) [0.00,6.00]
Total: 28620 W: 5042 L: 4798 D: 18780

bench: 7995098

src/main.cpp

@@ -37,6 +37,7 @@ int main(int argc, char* argv[]) {
   Position::init();
   Bitbases::init_kpk();
   Search::init();
+  Pawns::init();
   Eval::init();
   Threads.init();
   TT.set_size(Options["Hash"]);