Transform features per specified math function.
        :param y:
        :return:

def transform(self, y):
        """
        Transform features per specified math function.
        :param y:
        :return:
        """
        if isinstance(y, pd.DataFrame):
            x = y.ix[:,0]
            y = y.ix[:,1]
        else:
            x = y[:,0]
            y = y[:,1]
        if self.transform_type == 'add':
            return pd.DataFrame(np.add(x, y))
        elif self.transform_type == 'sub':
            return pd.DataFrame(np.subtract(x, y))
        elif self.transform_type == 'mul':
            return pd.DataFrame(np.multiply(x, y))
        elif self.transform_type == 'div':
            return pd.DataFrame(np.divide(x, y))
        elif self.transform_type == 'rem':
            return pd.DataFrame(np.remainder(x, y))
        elif self.transform_type == 'pow':
            return pd.DataFrame(x**y)