Do not copy master position in split()

A pointer is enough because after a split point has been
setup master and slaves thread end up calling sp_search() or
sp_search_pv() and here a full copy of split point position is
done again, note that even master does another copy (of itself)
and this is done before any do_move() call so that master Position
is never updated between split() and sp_search().

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

src/position.h

@@ -147,8 +147,8 @@ public:
 
   // Constructors
   Position();
-  Position(const Position& pos);
-  Position(const std::string& fen);
+  explicit Position(const Position& pos);
+  explicit Position(const std::string& fen);
 
   // Text input/output
   void from_fen(const std::string& fen);