Add psqt ordering when there is no history

This seems to increase strenght (about 15 ELO),
still to test some variations on this theme that
could increase ELO even more.

Idea from Rebel (http://members.home.nl/matador/chess840.htm)

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

src/position.h

@@ -278,6 +278,7 @@ public:
   Value eg_value() const;
   Value non_pawn_material(Color c) const;
   Phase game_phase() const;
+  Value mg_pst_delta(Move m) const;
 
   // Game termination checks
   bool is_mate();
@@ -681,6 +682,11 @@ inline Value Position::mg_pst(Color c, PieceType pt, Square s) const {
   return MgPieceSquareTable[piece_of_color_and_type(c, pt)][s];
 }
 
+inline Value Position::mg_pst_delta(Move m) const {
+  return MgPieceSquareTable[piece_on(move_from(m))][move_to(m)]
+        -MgPieceSquareTable[piece_on(move_from(m))][move_from(m)];
+}
+
 inline Value Position::eg_pst(Color c, PieceType pt, Square s) const {
   return EgPieceSquareTable[piece_of_color_and_type(c, pt)][s];
 }