docs: def find_nb_genes(data):
      """
      Finds the indices of all genes in the dataset that have
      a mean < 0.9 variance. Returns an array of booleans.
      """
      data_means = data.mean(1)
      data_vars = data.var(1)
      nb_indices = data_means < 0.9*data_vars
      return nb_indices