message: Use reference in call to scaled_sub in test
diff: //! // The method `gradient()` has been implemented by deriving `ArgminSolver`.
  //! let grad = self.gradient(&param)?;
  //! // Calculate new parameter vector based on update rule
  -//! let new_param = param.scaled_sub(self.omega, grad);
  +//! let new_param = param.scaled_sub(self.omega, &grad);
  //! // Return new parameter vector. Since there is no need to compute the cost function
  //! // value, we return 0.0 instead.
  //! let out = ArgminIterationData::new(new_param, 0.0);
  