New Imbalance Tables Tweak

Imbalance tables tweaked to contain MiddleGame and Endgame values, instead of a single value.

The idea started from Fisherman, which requested my help to tune the values back in June/July,
so I tuned the values back then, and we were able to accomplish good results,
but not enough to pass both STC and LTC tests.

So after the recent changes, I decided to give it another shot, and I am glad that it was a successful attempt.

A special thanks goes also to mstembera, which notified me a simple way to let the patch perform a little better.

Passed STC:
LLR: 2.94 (-2.94,2.94) {-0.25,1.25}
Total: 115976 W: 23124 L: 22695 D: 70157
Ptnml(0-2): 2074, 13652, 26285, 13725, 2252
https://tests.stockfishchess.org/tests/view/5fc92d2d42a050a89f02ccc8

Passed LTC:
LLR: 2.94 (-2.94,2.94) {0.25,1.25}
Total: 156304 W: 20617 L: 20024 D: 115663
Ptnml(0-2): 1138, 14647, 46084, 15050, 1233
https://tests.stockfishchess.org/tests/view/5fc9fee142a050a89f02cd3e

closes https://github.com/official-stockfish/Stockfish/pull/3255

Bench: 4278746

src/material.h

@@ -37,8 +37,8 @@ namespace Material {
 
 struct Entry {
 
-  Score imbalance() const { return make_score(value, value); }
-  Phase game_phase() const { return gamePhase; }
+  Score imbalance() const { return score; }
+  Phase game_phase() const { return (Phase)gamePhase; }
   bool specialized_eval_exists() const { return evaluationFunction != nullptr; }
   Value evaluate(const Position& pos) const { return (*evaluationFunction)(pos); }
 
@@ -57,9 +57,9 @@ struct Entry {
   const EndgameBase<Value>* evaluationFunction;
   const EndgameBase<ScaleFactor>* scalingFunction[COLOR_NB]; // Could be one for each
                                                              // side (e.g. KPKP, KBPsK)
-  int16_t value;
+  Score score;
+  int16_t gamePhase;
   uint8_t factor[COLOR_NB];
-  Phase gamePhase;
 };
 
 typedef HashTable<Entry, 8192> Table;