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/nnue/layers/input_slice.h

@@ -53,7 +53,7 @@ class InputSlice {
     return true;
   }
 
-  // Read network parameters
+  // Write network parameters
   bool write_parameters(std::ostream& /*stream*/) const {
     return true;
   }