Nino Tempo is accurately described as a Purveyor of Balladry on Omnivore Recordings’ new compilation dedicated to his Atlantic recordings, but that’s just one facet of this all-too-underrated artist.  Over a long career, Tempo has served as a writer, producer, arranger, singer, and (as on this set) musician.  With his sister April Stevens, Tempo enchanted with the breezy “Deep Purple” and shimmering “You’ll Be Needing Me Baby,” and out-Righteous Brother’ed the Righteous Brothers on “All Strung Out.”  A child actor and amateur hour winner, Tempo played with the illustrious “Wrecking Crew” on…