rs4pi: get max x range for DVH report
rs4pi: get max x range for DVH report

mkeilman/sirepo,radiasoft/sirepo,radiasoft/sirepo,mrakitin/sirepo,radiasoft/sirepo,mkeilman/sirepo,mrakitin/sirepo,mkeilman/sirepo,radiasoft/sirepo,mkeilman/sirepo,mrakitin/sirepo,radiasoft/sirepo,mrakitin/sirepo