Revision 11226

From Anna Kreshuk:
- TLinearFitter class: Linear fitter is used to fit a set of data points with a linear
  combination of specified functions (with or without weights). The "linear" in the name
  stands for "linear in parameters", not variables. The main advantages,
  compared to a general non-linear fitter, is that the linear fitter is considerably
  faster and doesn't require setting the initial values of parameters.
  The linear fitter can be used directly or through TH1::Fit and TGraph::Fit functions
  When it's used directly, functions of more than 3 dimensions can be fitted.

- TFormula: a new data member fLinearParts and methods to process it are added. Now
  one can create a function of the type that can be fit by the linear fitter.
  Example: TFormula f("f", "x++sin(x)") is equivalent to
  TFormula f("f","[0]*x + [1]*sin(x)"), but if a TH1::Fit or a TGraph::Fit is performed
  with the 1st formula, it goes through the linear fitter, if it's performed with the
  2nd formula, it goes through Minuit.

- TMultiGraph class: the Fit method is implemented, allowing to fit all the graphs
  of a multigraph simultaneously

- TGraph, TGraphErrors, TGraphAsymmErrors, TGraphBentErrors - functions GetEXhigh(),
  GetEXlow(), GetEYhigh(), GetEYlow() implemented to simplify the GraphFitChisquare()
  function of the TFitter class

- TH1, TGraph, TGraph2D, TMultiGraph - changes in the Fit functions, allowing to use
  the linear fitter. Fitting with "polN" functions redirected to use the linear fitter,
  in cases where no options, except "range", are specified.