“They are the quintessence of Bach’s work as a composer, and Bach is the quintessence of all music,” Pablo Casals said about the Baroque master’s Cello Suites. Written around 1720, the six works set new standards, both technically and musically, and remain a touchstone for any cellist to this day. Kian Soltani, frequently heard at the Pierre Boulez Saal in past seasons, leads audiences through this fascinating musical labyrinth on two consecutive nights.