Tweak stat bonus formula

Tweak stat bonus formula on top of latest elo gain by @snicolet

STC
http://tests.stockfishchess.org/tests/view/5b830a810ebc5902bdbb7e9c
LLR: 2.95 (-2.94,2.94) [0.00,4.00]
Total: 27797 W: 6113 L: 5842 D: 15842

LTC
http://tests.stockfishchess.org/tests/view/5b831f2c0ebc5902bdbb8038
LLR: 2.95 (-2.94,2.94) [0.00,4.00]
Total: 13655 W: 2294 L: 2099 D: 9262

I think that more elo can be found in tweaks of this parameters so I plan
to further try some "hand-tuning", including increasing/decreasing ratio of
two constants and making bonus assimetric to 0. Thx to @AndyGrant for helping
with github and @jerrydonaldwatson for original idea.

Closes #1748

Bench: 4172767

src/search.cpp

@@ -723,10 +723,10 @@ namespace {
     else
     {
         int p = (ss-1)->statScore;
-        int malus = p > 0 ? (p + 5000) / 1024 :
-                    p < 0 ? (p - 5000) / 1024 : 0;
+        int bonus = p > 0 ? (-p - 2500) / 512 :
+                    p < 0 ? (-p + 2500) / 512 : 0;
 
-        ss->staticEval = eval = (ss-1)->currentMove != MOVE_NULL ? (pureStaticEval = evaluate(pos)) - malus
+        ss->staticEval = eval = (ss-1)->currentMove != MOVE_NULL ? (pureStaticEval = evaluate(pos)) + bonus
                                                                  : (pureStaticEval = -(ss-1)->staticEval + 2 * Eval::Tempo);
 
         tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, pureStaticEval);