Tolstoy famously pronounced, “Every unhappy family is unhappy in its own way,” and the Odinsons, who rule a world called Asgard, are spectacularly dysfunctional.A jocular thread runs through “Thor: Ragnarok,” giving its star, Chris Hemsworth, a chance to flex his comedic muscles (as well as the others), and making him more relatable to bro-verse fans than the flowing-haired heartthrob of the first “Thor” movie.But the banter does not neutralize the film’s ultimately dispiriting …