Double Infinity is out September 5; the Somersault Slide 360 Tour starts September 17. Left to right: James Krivchenia, Adrianne Lenker, Buck Meek. Photo: Alexa Viscius For more than a decade, Brooklyn- by-way-of-everywhere trio Big Thief have put out cosmic indie-rock albums to near-universal acclaim. Live, the band departs unexpectedly from the studio script, and performances of the same songs can turn out very different on a given night. During a ten- minute performance of “Not” at the