In the playbill for the Steppenwolf Theatre production of Branden Jacobs-Jenkins’ world premiere of “Purpose,” artistic directors Audrey Francis and Glenn Davis announce that “since the beginning, family drama has been this company’s signature.”Indeed. “Purpose” follows the theater company’s tradition of family drama. At the same time, it’s unlike anything that Steppenwolf has done before.Directed by actress and director Phylicia Rashad, famous for playing Clair Huxtable on the hit 1980s TV sitcom “The Cosby Show,” has an …