The slyest aspect about Lone Scherfig’s “The Riot Club” is also its most maddening one. Structurally, the whole production is a come-on, a tease, a manipulative stunt. It begins in lusciously ribald fashion as we are treated to nineteenth-century sexual shenanigans amid the upper classes at Oxford University: white wigs, splashy capes and all. A beloved hedonist is murdered after cuckolding an older man; to pay heed to his decadence, his peers vow to start a club