Selects k best features in dataset

        :param k: features to select
        :return: k best features

def select_k_best(self, k):
        """Selects k best features in dataset

        :param k: features to select
        :return: k best features
        """
        x_new = SelectKBest(chi2, k=k).fit_transform(self.x_train, self.y_train)
        return x_new