İndexes of the K fold cross validation : https://github.com/enginbozaba/R_MyFunctions/blob/master/k_fold_cross_validation.r
Normalization : https://github.com/enginbozaba/R_MyFunctions/blob/master/normalization.r
Covariance : https://github.com/enginbozaba/R_MyFunctions/blob/master/covariance.r