Doxygen for some more hist/hist classes

documentation/doxygen/images/tgraph2d_delaunay.gif

hist/hist/inc/TFormula.h

hist/hist/inc/TFractionFitter.h

hist/hist/src/TF1.cxx

hist/hist/src/TF12.cxx

hist/hist/src/TF1NormSum.cxx

hist/hist/src/TF3.cxx

hist/hist/src/TFitResult.cxx

hist/hist/src/TFitResultPtr.cxx

hist/hist/src/TFormula.cxx

hist/hist/src/TFractionFitter.cxx

hist/hist/src/TGraph.cxx

hist/hist/src/TGraph2D.cxx

hist/hist/src/TGraph2DErrors.cxx

hist/hist/src/TGraphAsymmErrors.cxx

hist/hist/src/TGraphBentErrors.cxx

hist/hist/src/TGraphDelaunay.cxx

hist/hist/src/TGraphErrors.cxx

hist/hist/src/TGraphSmooth.cxx

hist/hist/src/TGraphTime.cxx