Martin Puryear’s C.F.A.O., 2006-2007, painted and unpainted pine and found wheelbarrow. Courtesy the artist and Donald Young Gallery, Chicago. © 2008 Martin Puryear. Photo Richard P. Goodbody When Martin Puryear visited the National Gallery of Art last week for the press opening of his retrospective show, he spoke about how he grew up in Washington, and as a child would often visit the gallery. He didn’t imagine that one day the museum would host…