“Il Trovatore” fits awkwardly in Verdi’s extraordinarily fruitful “middle period.”  Wedged between the musical and dramatic innovations of “Rigoletto” and “La Traviata,” it seems to be a conservative throwback to an Italian idiom established by Rossini and his early successors.   It is a “numbers” opera with a melodramatic plot (based on Antonio Garcia Gutiérrez’s “El trovador”) that beggars belief. {…}