The Ice Storm, which opened the prestigious New York Film Festival on Sept. 26, is the best film about family so far this year. Just don't think Disney, since the parents indulge in adultery and wife swapping, and the kids, ages 14 to 16, dry hump, mix drug cocktails and contemplate suicide. The setting is the plush suburb of New Canaan, Conn. The time is Thanksgiving weekend of 1973. Nixon drones denials from every TV, and in every room, children can hear their parents ripping into each other