code: public void setLevels(List<String> levels, boolean isOrdered) throws DDFException {
      this.setLevels(levels, null, isOrdered);
    }
docs: Typically, levels are automatically computed from the data, but in some rare instances, the user may want to
  specify the levels explicitly, e.g., when the data column does not contain all the levels desired.
  
  @param levels
  @param isOrdered a flag indicating whether the levels actually have "less than" and "greater than" left-to-right order
  meaning
  @throws DDFException