Use ply counter in Position object

And avoid a redundant one passed as argument in
search calls.

Also renamed gamePly in ply to better clarify this
is used as search ply and is set to zero at the
beginning of the search.

No functional change.

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

src/uci.cpp

@@ -206,7 +206,7 @@ namespace {
                 move = move_from_string(RootPosition, token);
                 RootPosition.do_move(move, st);
                 if (RootPosition.rule_50_counter() == 0)
-                    RootPosition.reset_game_ply();
+                    RootPosition.reset_ply();
             }
             // Our StateInfo st is about going out of scope so copy
             // its content inside RootPosition before they disappear.