The unveiling of a retractable roof on Jersey City’s Pershing Field swimming pool was postponed Wednesday morning because of the rain. But the release of the annual national rankings of municipal park systems managed to brighten the day for city officials. Jersey City rose 10 places to 36th among the nation’s 100 largest cities in theTrust for Public Land’s ParkScore Index. Washington, D.C. was No. 1, followed closely by St. Paul, Minnesota. Newark dropped slightly to 47th. City officials were pleased, but not necessarily surprised after creating open space and improving existing parks has bee…