docs: def output_clusters(labels, cluster_centers_indices):
      """Write in tab-separated files the vectors of cluster identities and
          of indices of cluster centers.
      """
              
      here = os.getcwd()
      try:
          output_directory = os.path.join(here, 'concurrent_AP_output')
          os.makedirs(output_directory)
      except OSError:
          if not os.path.isdir(output_directory):
              print("ERROR: concurrent_AP: output_clusters: cannot create a directory "
                    "for storage of the results of Affinity Propagation clustering "
                    "in your current working directory")
              sys.exit(1)
              
      if any(np.isnan(labels)):
          fmt = '%.1f'
      else:
          fmt = '%d'
                  
      with open(os.path.join(output_directory, 'labels.tsv'), 'w') as fh:
          np.savetxt(fh, labels, fmt = fmt, delimiter = '\t')
          
      if cluster_centers_indices is not None:    
          with open(os.path.join(output_directory, 'cluster_centers_indices.tsv'), 'w') as fh:
              np.savetxt(fh, cluster_centers_indices, fmt = '%.1f', 
                         delimiter = '\t')