Even just a decade or two ago, classical musicians might look back occasionally to the Baroque era or try out a new work, but most stuck to Johannes Brahms, Franz Schubert, and the genre’s other tried-and-true standard bearers. But today, many of the field’s younger generation of artists, who can access music from virtually any time or place in seconds on their iPhones, don’t feel nearly so confined. They might play a quartet by Ludwig van Beethoven one night and then join forces with an indie-rock band the next.