Essential by Christophe began serving dinner on Sunday night a few weeks ago, making it the only restaurant in its high-priced class open seven nights a week. Demand for seats left chef/owner Christophe Bellanca no other choice.  It’s not only the best restaurant on the Upper West Side — a neighborhood not known as a culinary nirvana — it’s my favorite French restaurant of any kind in New York since Daniel opened in 1999, if not since Le Bernardin opened in 1986. Less than three years after it