Thought to be some of the most beautiful and important compositions ever written for cello, Bach’s suites for unaccompanied cello are a collection of dances, each prefaced by a searching prelude. Cellist István Várdai makes his North American recital debut at the Armory in a survey of these lauded works, from the open serenity of the G major to the powerful radiance of the D major, known as a “symphony for solo cello.”