class_docstr: Reduce corpus to the X most important terms.  Great for plotting a big corpus.
  Will return between X/2 and X terms.
  
  Returns the terms with the X highest absolute scores, background corpus scores, and term frequencies.
code: def _get_background_terms(num_term_to_keep, term_doc_matrix):
      return (
          term_doc_matrix.get_scaled_f_scores_vs_background()
          .iloc[: int(0.375 * num_term_to_keep)]
          .index
      )
  
label_desc: Use image node_id 4 for calling the AutoTermSelector obj's underlying member method code with example usage: obj._get_background_terms(num_term_to_keep, term_doc_matrix) and returns: term_doc_matrix