Simplify color usage in search.

define Color us and use this instead of pos.side_to_move() and nmp_odd. The latter allows to clarify the nmp verification criterion.

Tested for no regression:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 76713 W: 15303 L: 15284 D: 46126
http://tests.stockfishchess.org/tests/view/5b046a0d0ebc5914abc12971

No functional change.

src/thread.h

@@ -61,7 +61,8 @@ public:
   Material::Table materialTable;
   Endgames endgames;
   size_t PVIdx, PVLast;
-  int selDepth, nmp_ply, nmp_odd;
+  int selDepth, nmp_min_ply;
+  Color nmp_color;
   std::atomic<uint64_t> nodes, tbHits;
 
   Position rootPos;