CHICAGO — If you stop by Chicago’s Field Museum right now and find yourself in the Alsdorf Hall of Northwest Coast and Arctic Peoples, or the Robert R. McCormick Halls of the Ancient Americas, you will notice something about the display cases: Several are covered up. That in itself is not unusual — who hasn’t been to a museum and seen a display case displaying nothing? What’s unusual is the reason: On Jan. 12, federal regulations concerning the exhibition and study of Native American remains and sacred artifacts were tightened, to bring teeth and clarity to a set of rules that languished for …