The creators of "Urinetown," the most disgustingly titled musical ever to be an improbable Broadway success, come to Chicago with what they hope will be a close runner-up. "Yeast Nation," set in 3,000,458,000 BC amid a tribe of single-celled organisms, is a tale of revolution with a gooey '80s score. "Yeast" makes a few Chicago critics gag, but the majority think it's just delicious.