Supermarket group Asda is looking to extend the "mary-kateandashley" branded range to include books, DVDs, computer games and possibly even a range of clothing for boys on the back of the range's success in the UK.   The store chain, which is owned by US parent Wal-Mart Stores Inc, launched the girls' clothing range from US "tween" queens Mary-Kate and Ashley Olsen six weeks ago - and says sales have rocketed to more than £1 million.