The museum???s ambitious multi-part exhibition, Changing Landscapes, Changing Visions, begins this month with two related shows featuring such artists as Georgia O???Keeffe, Charles E. Burchfield, Carey Maxon, George Inness, John James Audubon and Louis Comfort Tiffany. ???Nature and Spirit: American Art of the 19th and 20th Centuries??? offers 25 paintings, pieces of sculpture, ceramics and decor