This week, the a cappella vocal group Pentatonix notched its first-ever number one on the Billboard Adult Contemporary chart.  More remarkably, the song was first recorded in 1937, written by a composer-lyricist who died in 1989, and features a vocalist who passed away in 1998.  The song is “I’ve Got My Love to Keep Me Warm,” the songwriter is Irving Berlin, and the duet partner is, of course, Frank Sinatra.  With this achievement, Sinatra returned to the top of the AC chart for the first time since September 30, 1967, when “The…