Add eg component to evaluate_shelter() #2137

Add an endgame component to the blockedstorm penalty
so that the penalty applies more uniformly through the game.

STC 10+0.1 th 1 :
LLR: -2.95 (-2.94,2.94) [0.50,4.50]
Total: 94063 W: 21426 L: 21118 D: 51519
http://tests.stockfishchess.org/tests/view/5cd4605c0ebc5925cf04bf43

LTC 60+0.6 th 1 :
LLR: 2.95 (-2.94,2.94) [0.00,3.50]
Total: 188232 W: 32808 L: 32090 D: 123334
http://tests.stockfishchess.org/tests/view/5cd47d0a0ebc5925cf04c4fd

Refactored code with higher constant values gave a more convincing LTC result:
LLR: 2.96 (-2.94,2.94) [0.00,3.50]
Total: 30050 W: 5330 L: 5066 D: 19654
http://tests.stockfishchess.org/tests/view/5cd6a0000ebc5925cf050653

Bench: 3687700

src/pawns.h

@@ -50,7 +50,7 @@ struct Entry {
   Score do_king_safety(const Position& pos);
 
   template<Color Us>
-  Value evaluate_shelter(const Position& pos, Square ksq);
+  void evaluate_shelter(const Position& pos, Square ksq, Score& shelter);
 
   Key key;
   Score scores[COLOR_NB];