merge update 45873 from roostats dev branch...

merge update 45873 from roostats dev branch (https://root.cern.ch/svn/root/branches/dev/roostats) done by George Lewis

Updated histfactory so models can be built without a data tag selected


git-svn-id: http://root.cern.ch/svn/root/trunk@45894 27541ba8-7e3a-0410-8455-c3a389f83636