Leonard Slatkin has been programming good music for the National Symphony Orchestra this season, much of it featuring an impressive roster of guest soloists. So far this fall, this DCist has heard Itzhak Perlman playing Barber and could have heard, but sadly missed, Nikolaj Znaider playing Bruch and Truls Mørk playing Elgar. True, more unusual fare has been safely packaged with favorites, for the most part, but in concert programming, just as in politics, it…