Add new data members

  Bool_t fGraphicsMode;
  TObject *fPlot;

By default fGraphicsMode is true.
When calling the Minuit functions such as mncont, mnscan, or any Minuit
command invoking mnplot, TMinuit::mnplot produces a TGraph object
pointed by TObject *fPlot.
One can retrieve this object with TMinuit::GetPlot();
For example, one can do:
  h->Fit("gaus");
  gMinuit->Command("SCAn 1");
  TGraph *gr = (TGraph*)gMinuit->GetPlot();
  gr->setMarkerStyle(21);
  gr->Draw("alp");

To set Minuit in no graphics mode, call gMinuit->SetGraphicsMode(kFALSE);


git-svn-id: http://root.cern.ch/svn/root/trunk@5539 27541ba8-7e3a-0410-8455-c3a389f83636