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

def transform(self, y):
        """
        Transform features per specified math function.
        :param y:
        :return:
        """
        if self.transform_type == 'log':
            return np.log(y)
        elif self.transform_type == 'exp':
            return np.exp(y)
        elif self.transform_type == 'sqrt':
            return np.sqrt(y)
        elif self.transform_type == 'sin':
            return np.sin(y)
        elif self.transform_type == 'cos':
            return np.cos(y)
        elif self.transform_type == 'tan':
            return np.tan(y)
        elif self.transform_type == 'abs':
             return np.abs(y)