For three-quarters of a century, the lonely farm couple depicted in Grant Wood’s ubiquitous painting, “American Gothic,” have called Chicago’s downtown Loop home, an incongruous base of operations for such a distinctly rural pair. But this fall, in a rare case in which the Art Institute of Chicago has consented to lend out one of its prized works, the painting will be going “home” to Iowa for a special Wood retrospective at the Cedar Rapids Museum of Art. It may be just another painting in Chicago, but Iowa is already abuzz with talk of the return.