Washington National Opera has made another significant advance in catching up to the latest trends in opera houses around the world, by staging its first-ever opera by Christoph Willibald Gluck (1714-1787). The new production of Iphigénie en Tauride, which opened on Friday night, turns out to be the best work in an otherwise downsized and somewhat ho-hum season — not only because it is the company’s first Gluck opera and such a beautiful work, but because of a superb cast that proves gripping both musically and dramatically, in a production that is intriguing, stark and far from ordinary.