Ever since violinist David Balakrishnan and cellist Mark Summer founded the Turtle Island Quartet in 1985, the ensemble has devoted its repertoire to breaking boundaries. The musicians’ latest excursion into virgin territories finds them teaming with Cajun fiddler Michael Doucet in a swinging, grooving program they built around “Louisiana Story,” the 1948 film with a Pulitzer Prize-winning score by American composer Virgil Thompson.