György Ligeti The 18 Etudes Danny Driver Hyperion   Composed between 1985 and 2001, the 18 Etudes by György Ligeti are an eloquent summary of the techniques he had developed throughout his career. They rival the best collections of etudes for piano while adding substantially to the variety of technical means to be explored, particularly in the realms of polyrhythm and sonority.    There are a number of recordings of the Etudes and it is difficult to choose a favorite: different ones excel at various aspects of these multifaceted works. Danny Driver’s is a strong contender. Amply powerful where