message: error handling for set_gamma function
diff: @@ -41,9 +41,15 @@ where
  }
  /// set gamma
  - pub fn set_gamma(&mut self, gamma: f64) -> &mut Self {
  + pub fn set_gamma(&mut self, gamma: f64) -> Result<&mut Self, Error> {
  + if gamma <= 0.0 || gamma > 1.0 {
  + return Err(ArgminError::InvalidParameter {
  + text: "Newton: gamma must be in (0, 1].".to_string(),
  + }
  + .into());
  + }
  self.gamma = gamma;
  - self
  + Ok(self)
  }
  }
  