February, in my memory, was the month of the worst Nor’easters, bringing snows four feet deep drifting to 15. This February, the San Francisco Symphony Chorus is rallying as the result of a different sort of perfect storm: Ligeti’s Requiem. The 20-part, 20-minute piece, known best for its use in 2001, A Space Odyssey, ranks as one of the most difficult the Chorus has prepared.