docs: def add_qtl_to_map(qtlfile, mapfile, outputfile='map_with_qtls.csv'):
      """ This function adds to a genetic map for each marker the number
      of significant QTLs found.
  
      :arg qtlfile, the output from MapQTL transformed to a csv file via
          'parse_mapqtl_file' which contains the closest markers.
      :arg mapfile, the genetic map with all the markers.
      :kwarg outputfile, the name of the output file in which the map will
          be written.
  
      """
      qtl_list = read_input_file(qtlfile, ',')
      map_list = read_input_file(mapfile, ',')
      map_list[0].append('# QTLs')
      markers = []
      markers.append(map_list[0])
      qtl_cnt = 0
      for marker in map_list[1:]:
          markers.append(add_qtl_to_marker(marker, qtl_list[1:]))
          qtl_cnt = qtl_cnt + int(markers[-1][-1])
      LOG.info('- %s markers processed in %s' % (len(markers), mapfile))
      LOG.info('- %s QTLs located in the map: %s' % (qtl_cnt, outputfile))
      write_matrix(outputfile, markers)