- Add possibility to specify a User fitting function in TGraph::Fit:

  By default the fitting function GraphFitChisquare is used.
  To specify a User defined fitting function, specify option "U" and
  call the following functions:
    TVirtualFitter::Fitter(mygraph)->SetFCN(MyFittingFunction)
  where MyFittingFunction is of type:
  extern void MyFittingFunction(Int_t &npar, Double_t *gin, Double_t &f, Double_t *u, Int_t flag);


git-svn-id: http://root.cern.ch/svn/root/trunk@59 27541ba8-7e3a-0410-8455-c3a389f83636