code: def column_name(self, source, allele_group):
          
          return self.column_format.format(
              source=source,
              allele_group=allele_group)
docs: Parameters
          ----------
          source : string
              name of the ReadSource
  
          allele_group : string
              one of: num_ref, num_alt, total_depth
  
          Returns
          ----------
          column name : string