Change evaluation GrainSize from 4 to 8

Idea from Joona.

After 999 games at 1+0 on my Intel Core 2 Duo
Orig - Mod: +215 =538 -226 (+11 ELO)

On Joona QUAD after 845 games at 1+0
Orig - Mod: 151 - 181 - 513 (+13 elo)

So it seems a good change !

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/evaluate.cpp

@@ -43,7 +43,7 @@ namespace {
   const int Sign[2] = { 1, -1 };
 
   // Evaluation grain size, must be a power of 2
-  const int GrainSize = 4;
+  const int GrainSize = 8;
 
   // Evaluation weights, initialized from UCI options
   int WeightMobilityMidgame, WeightMobilityEndgame;