Implement new methods in TGenerator (suggested by Christian Holm)

	virtual void            GenerateEvent();
        virtual Double_t        GetParameter(const char* /*name*/) const { return 0.; }
        virtual void            SetParameter(const char* /*name*/,Double_t /*val*/){}


git-svn-id: http://root.cern.ch/svn/root/trunk@14797 27541ba8-7e3a-0410-8455-c3a389f83636