IN HIS SONGS, Nick Cave descends to the catacombs of the imagination, drawing on myth, mysticism and religion. You might encounter Lazarus or Orpheus, Old Testament fury or glory, and always lurking, the threat of damnation, the sweet promise of salvation. Yet, as Cave says in this new film (more on what type of film it might be in a moment), and explained in an interview some years back with Peter Stanford in these pages, his interest in religion is largely confined to his work. He may look like a hellfire preacher but that is not the life. It is tempting to describe 20,000 Days on Earth as a documentary about Cave’s work. Ostensibly it portrays – with liberal zigzags of time and space – a day in his life, around the recording of his last album. Yet, rather than a &ldqu