changed xMax/zMin defaults, refactored field visible/enabled states
changed xMax/zMin defaults, refactored field visible/enabled states

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