docs: def get_quality_report_string(self):
      """get a report on quality score distribution.  currently prints to stdout"""
      if not self._quality_distro:
        self.analyze_quality()
      ostr = ""
      for type in sorted(self._quality_distro.keys()):
        total = sum([ord(x)*self._quality_distro[type][x] for x in self._quality_distro[type]])
        cnt = sum([self._quality_distro[type][x] for x in self._quality_distro[type]])
        if cnt == 0: continue
        print 'type: '+type+' '+str(cnt)+' '+str(float(total)/float(cnt))
      return ostr