By Denny Dyroff, Staff Writer, The Times David Torn, who performs on May 14 at Philadelphia Art Alliance, might just be the best, least-known guitar player in the world, if not the most versatile. There is a huge array of genres in popular music today and, within each of those genres are many excellent guitarists. There are rock greats such as Jeff Beck, Eric Clapton and Derek Trucks. There are jazz...