Replace Position::copy()

With assignment operator. And fix Position::flip().

No functional change.

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

src/benchmark.cpp

@@ -107,7 +107,7 @@ void benchmark(istringstream& is) {
 
   for (size_t i = 0; i < fens.size(); i++)
   {
-      Position pos(fens[i], false, NULL);
+      Position pos(fens[i], false, Threads.main_thread());
 
       cerr << "\nPosition: " << i + 1 << '/' << fens.size() << endl;