Ethan Hawke’s first feature documentary “Seymour: an introduction” makes no attempt to be comprehensive about the life of its titular subject, classical pianist Seymour Bernstein. Instead, it focuses on what drew Hawke to Seymour: his sagaciousness and reflections on music, artistic devotion, and life. Eighty-seven years young, Seymour is a guru who not only mentors in piano playing but also