Persist Ethel and Allison Loggins-Hull Sono Luminus   The string quartet Ethel presents a characteristically diverse program of contemporary music on Persist, their first recording for Sono Luminus. They are joined by composer/flutist Allison Loggins-Hull and the resulting quintet are strong advocates for the emerging composers featured here.   The title work is by Loggins-Hull, currently a composer fellow with the Cleveland Orchestra. Her work is gracefully written and appealing. Persist begins with an ambling section with an angular flute melody, pizzicato strings and percussion instruments. This is varied throughout, juxtaposed with presto passages featuring quickfire flute lines accompanied by