Function dt_attr_cmp

* Compare two attributes and return an integer value in the following ranges: * * <0 if any of a1's attributes are less than a2's attributes * =0 if all of a1's attributes are equal to a2's attributes * >0 if all of a1's attributes are greater than or equal to a2's attributes * * To implement this function efficiently, we subtract a2's attributes from * a1's to obtain a negative result if a