docs: protected void addSubreportsToGroups() {
          for (Integer groupNum : groupFooterSubreports.keySet()) {
              List<Subreport> list = groupFooterSubreports.get(groupNum);
              DJGroup group = report.getColumnsGroups().get(groupNum - 1);
              group.getFooterSubreports().addAll(list);
          }
  
          for (Integer groupNum : groupHeaderSubreports.keySet()) {
              List<Subreport> list = groupHeaderSubreports.get(groupNum);
              DJGroup group = report.getColumnsGroups().get(groupNum - 1);
              group.getHeaderSubreports().addAll(list);
          }
  
      }