Plot: Introduced by Jane Glover Dance - the theme of the 93rd season of Henry Wood Promenade concerts - is a subject close to the heart of tonight's conductor, Isaiah Jackson , who has worked with the Dance Theatre of Harlem and next month becomes the music director of the Royal Ballet. The main work in tonight's concert is Copland: Dance Symphony, described by its composer as 'one of the things-called-symphonies-that-aren't'