Add an UCI level command "export_net".

This command writes the embedded net to the file `EvalFileDefaultName`.
If there is no embedded net the command does nothing.

fixes #3453

closes https://github.com/official-stockfish/Stockfish/pull/3454

No functional change

src/evaluate.h

@@ -45,6 +45,7 @@ namespace Eval {
     Value evaluate(const Position& pos);
     bool load_eval(std::string name, std::istream& stream);
     void init();
+    void export_net();
     void verify();
 
   } // namespace NNUE