code: def promoter(self, up=2000, down=0):
          
          if not self.is_gene_pred: return None
          return self.tss(up=up, down=down)
docs: Return a start, end tuple of positions for the promoter region of this
          gene
  
          Parameters
          ----------
  
          up : int
             this distance upstream that is considered the promoter
  
          down : int
             the strand is used to add this many downstream bases into the gene.