message: added Hessian type
diff: @@ -18,6 +18,7 @@ struct MyProblem {}
  impl ArgminOperator for MyProblem {
  type Parameters = Vec<f64>;
  type OperatorOutput = Vec<f64>;
  + type Hessian = ();
  fn apply(&self, p: &Vec<f64>) -> Result<Vec<f64>, Error> {
  Ok(vec![4.0 * p[0] + 1.0 * p[1], 1.0 * p[0] + 3.0 * p[1]])
  