message: updated writer example
diff: @@ -14,6 +14,7 @@ use argmin::testfunctions::rosenbrock;
  use argmin_core::finitediff::*;
  use ndarray::{array, Array1, Array2};
  use serde::{Deserialize, Serialize};
  +use std::sync::Arc;
  #[derive(Clone, Default, Serialize, Deserialize)]
  struct Rosenbrock {
  @@ -56,8 +57,14 @@ fn run() -> Result<(), Error> {
  // Attach a logger
  solver.add_logger(ArgminSlogLogger::term());
  + // Create writer
  + let mut writer = WriteToFile::new("params");
  +
  + // Set serializer to JSON
  + writer.set_serializer(WriteToFileSerializer::JSON);
  +
  // Attach a writer
  - solver.add_writer(WriteToFile::new("params"));
  + solver.add_writer(Arc::new(writer));
  // Set how often parameter should be saved
  solver.set_writer_mode(WriterMode::Every(3));
  