Linear regression with L2 regularization.
    Syntax
      parameters = ridgefit(X,y)
      parameters = ridgefit(X,y,options)
    Inputs
      
      
        
          - X
 
          - Training data.
 
          - Type: double
 
          - Dimension: vector | matrix
 
        
        
          - y
 
          - Target values.
 
          - Type: double
 
          - Dimension: vector | matrix
 
        
		
          - options
 
          - Type: struct
 
          - 
            
			  
                - to_normalize
 
                - If true, input data X will be normalized before performing regression (default: false). It is done by subtracting mean and dividing by the standard deviation.
 
                - Type: Boolean
 
				- Dimension: logical
 
              
			  
                - l2_penalty
 
                - Regularization Strength (default: 0). If value greater than 0, it becomes Ridge Regression.
 
                - Type: double | integer
 
				- Dimension: scalar
 
              
            
           
        
      
      
    
    Outputs
      
      
        
          - parameters
 
          - Contains all the values passed to ridgefit method as options. Additionally it has below key-value pairs.
 
		  - Type: struct
 
		  - 
            
              
                - scorer
 
                - Function handle pointing to r2 function (R2 Coefficient of Determination).
 
                - Type: function handle
 
              
			  
                - intercept
 
                - Estimated intercept.
 
                - Type: integer
 
				- Dimension: scalar
 
              
			  
                - coef
 
                - Estimated coefficients
 
                - Type: double
 
				- Dimension: vector
 
              
			  
                - params
 
                - Contains both intercept and coef as a vector. It is used by predict method.
 
                - Type: double
 
				- Dimension: vector
 
              
			  
                - n_samples
 
                - Number of rows in the training data.
 
                - Type: integer
 
				- Dimension: scalar
 
              
			  
                - n_features
 
                - Number of columns in the training data.
 
                - Type: integer
 
				- Dimension: scalar
 
              
            
           
        
      
      
    
    Example
      
      Usage of ridgefit with options
      X = [1 2 3; 4 5 6; 7 8 9; 10 11 12; 13 14 15; 16 17 18; 19 20 21];
y = [1, 2, 3, 4, 5, 6, 7];
options = struct;
options.to_normalize = true;
options.l2_penalty = 0.03;
parameters = ridgefit(X, y, options);
      > parameters
parameters = struct [
  coef: [Matrix] 3 x 1
  0.66572
  0.66572
  0.66572
  intercept: 4
  l2_penalty: 0.03
  n_features: 3
  n_samples: 7
  params: [Matrix] 4 x 1
  4.00000
  0.66572
  0.66572
  0.66572
  scorer: @r2
  to_normalize: 1
]
    
	Comments
      
      Output 'parameters' can be passed to ridgepredict function.