Jem Aswad Senior Music EditorSince the Beatles split in 1970, seemingly every known scrap of their history has been scrutinized and curated for public consumption — every minute of studio tape, every radio broadcast, home and concert recording; every photo and document and snippet of film footage — with one huge exception: the “Let It Be” film.There are several reasons for this, but only one matters: “Let It Be” is a downer.