message: fixed hagerzhang example
diff: @@ -19,6 +19,7 @@ struct MyProblem {}
  impl ArgminOperator for MyProblem {
  type Parameters = Vec<f64>;
  type OperatorOutput = f64;
  + type Hessian = ();
  fn apply(&self, param: &Vec<f64>) -> Result<f64, Error> {
  Ok(sphere(param))
  