Add new member functions to add, multiply or divide an histogram

with a function:
    virtual void     Add(TF1 *h1, Double_t c1=1);
    virtual void     Divide(TF1 *h1, Double_t c1=1);
    virtual void     Multiply(TF1 *h1, Double_t c1=1);


git-svn-id: http://root.cern.ch/svn/root/trunk@455 27541ba8-7e3a-0410-8455-c3a389f83636