Concert with a key
Kirill GersteinRarely has a performance been curated with such subtle thematic intent as Kirill Gerstein's at Celebrity Series last weekend.Its calling card was the Boston premiere of wunderkind Timo Andres' "Old Friend," a kind of millennial fantasia on Chopin's Third Scherzo. You know the Chopin - it's a dazzler split between two apparent emotional poles, one grumbling at the bottom of the keyboard, the other…