code: private String[] getHeaderTokens(Relation frame) {
          final int colCount = frame.columnCount();
          final String[] header = new String[colCount];
          IntStream.range(0, colCount).forEach(colIndex -> {
              header[colIndex] = frame.column(colIndex).name();
          });
          return header;
      }
docs: Returns the header string tokens for the frame
  
  @param frame the frame to create header tokens
  @return the header tokens