Implement a request from ATLAS:

In addition to the existing function
    virtual       TArrayD *GetSumw2()       {return &fSumw2;}
provide also the const vesrion
    virtual const TArrayD *GetSumw2() const {return &fSumw2;}


git-svn-id: http://root.cern.ch/svn/root/trunk@13713 27541ba8-7e3a-0410-8455-c3a389f83636