Washington National Opera is celebrating its 50th anniversary season this year, at the same time as the National Symphony Orchestra marks its 75th. Last night, WNO inaugurated its golden anniversary year by the premiere of a lesser-known opera, I Vespri Siciliani (The Sicilian Vespers), by a great and justly famous composer, Giuseppe Verdi. As an opera never mounted in the company’s history, it was an odd choice, since it is neither an old-guard favorite…