I was ready to be a little disappointed when I arrived at the Harris Theater for the Mark Morris Dance Group's performance of Dido and Aeneas last night. I knew that Morris himself had danced the piece's twin lead roles of Dido and the Sorceress at its premiere in 1989 and numerous times since, but had never seen it. And now that he wasn't taking the role himself anymore, I knew he had cast it with a woman. That sounded like it would be less interesting. I needn't have worried. There's nothing uninteresting about Laurel Lynch, who has both lead parts.