message: changed some default types
diff: @@ -46,7 +46,7 @@ pub struct GradientDescent<'a> {
  /// Indicates the current state of the Gradient Descent method.
  struct GradientDescentState<'a> {
  /// Reference to the problem. This is an Option<_> because it is initialized as `None`
  - problem: Option<&'a ArgminProblem<'a, Array1<f64>, f64, Array1<f64>>>,
  + problem: Option<&'a ArgminProblem<'a, Array1<f64>, f64, ()>>,
  /// Previous parameter vector
  prev_param: Array1<f64>,
  /// Current parameter vector
  @@ -139,7 +139,7 @@ impl<'a> GradientDescent<'a> {
  impl<'a> ArgminSolver<'a> for GradientDescent<'a> {
  type Parameter = Array1<f64>;
  type CostValue = f64;
  - type Hessian = Array1<f64>;
  + type Hessian = ();
  type StartingPoints = Array1<f64>;
  type ProblemDefinition = &'a ArgminProblem<'a, Self::Parameter, Self::CostValue, Self::Hessian>;
  