A night like this could get to be a habit. Saturday’s simulcast of San Francisco Opera’s Il trovatore was the seventh free, live simulcast in a tradition instituted by General Director David Gockley in 2006 and the fourth to be held at AT&T Park. The staid glamour of the opera house was happily exchanged by many for a jovial, picnic setting and a ticket price that can’t be beat. (It’s free.)