Improved handling of global observables

  o RooAbsPdf

    - Document already existing named argument GlobalObservablesTag() as alternative way to indicate
      global observables: all observables with a named bool attribute matching the specified tag
      will be considered as global observables

   - Add concept of 'default' global observables speficication: if the head node of a pdf
     contains a string attribute named 'DefaultGlobalObservablesTag' its value will be interpreted
     as the default argument to GlobalObservablesTag()


git-svn-id: https://root.cern.ch/svn/root/branches/v5-34-00-patches@49289 27541ba8-7e3a-0410-8455-c3a389f83636