Introduce king infiltration bonus

Introduce king infiltration bonus

Add king infiltration bonus to initiative calculation. Idea is somewhat similar
to outflanking - endgames are hard to win if each king is on it side of the board.
So this adds extra bonus for one of kings crossing the middle line.

STC
LLR: 2.94 (-2.94,2.94) {-1.00,3.00}
Total: 10533 W: 2372 L: 2242 D: 5919
Ptnml(0-2): 196, 1198, 2352, 1316, 202
http://tests.stockfishchess.org/tests/view/5e0e6fd1e97ea42ea89da9b3

LTC
LLR: 2.96 (-2.94,2.94) {0.00,2.00}
Total: 15074 W: 2563 L: 2381 D: 10130
Ptnml(0-2): 118, 1500, 4111, 1663, 129
http://tests.stockfishchess.org/tests/view/5e0e857ae97ea42ea89da9cc

Closes https://github.com/official-stockfish/Stockfish/pull/2471

Bench: 5146339