code: def gt_bases(self):
          
          result = []
          for a in self.gt_alleles:
              if a is None:
                  result.append(None)
              elif a == 0:
                  result.append(self.site.REF)
              else:
                  result.append(self.site.ALT[a - 1].value)
          return tuple(result)
docs: Return the actual genotype bases, e.g. if VCF genotype is 0/1,
          could return ('A', 'T')