Small clean-ups

- Comment for Countemove pruning -> Continuation history
- Fix comment in input_slice.h
- Shorter lines in Makefile
- Comment for scale factor
- Fix comment for pinners in see_ge()
- Change Thread.id() signature to size_t
- Trailing space in reprosearch.sh
- Add Douglas Matos Gomes to the AUTHORS file
- Introduce comment for undo_null_move()
- Use Stockfish coding style for export_net()
- Change date in AUTHORS file

closes https://github.com/official-stockfish/Stockfish/pull/3416

No functional change

src/thread.h

@@ -55,7 +55,7 @@ public:
   void idle_loop();
   void start_searching();
   void wait_for_search_finished();
-  int id() const { return idx; }
+  size_t id() const { return idx; }
 
   Pawns::Table pawnsTable;
   Material::Table materialTable;