If the industry experts interviewed in “Side by Side” are correct, children born today will graduate high school at a time when films are no longer shot on film. An industry standard for more than a century, celluloid is on the verge of being squeezed out of the business by digital recording. “Side by Side,” which makes its county debut Sunday at the Nevada Theatre, explores the history and future of filmmaking by examining the new options available to artists as technologies have changed and advanced. The documentary features talking-head interviews with some of the giants of cinema — Martin …