message: ObserverMode now on executor level
diff: @@ -52,23 +52,19 @@ fn run() -> Result<(), Error> {
  // Create writer
  let writer = WriteToFile::new("params", "param")
  - // Only save every 3 iterations
  - .mode(WriterMode::Every(3))
  // Set serializer to JSON
  .serializer(WriteToFileSerializer::JSON);
  // Create writer which only saves new best ones
  let writer2 = WriteToFile::new("params", "best")
  - // Only save new best
  - .mode(WriterMode::NewBest)
  // Set serializer to JSON
  .serializer(WriteToFileSerializer::JSON);
  let res = Executor::new(cost, solver, init_param)
  .max_iters(10)
  - .add_observer(ArgminSlogLogger::term())
  - .add_observer(writer)
  - .add_observer(writer2)
  + .add_observer(ArgminSlogLogger::term(), ObserverMode::Always)
  + .add_observer(writer, ObserverMode::Every(3))
  + .add_observer(writer2, ObserverMode::NewBest)
  .run()?;
  // Wait a second (lets the logger flush everything before printing again)
  