fix ROOT-5241 by implementing the option 'noclear'.
Implement a new function TH1;:Chisquare to compute the chi-square of an histogram given a function
(see ROOT-4722)

Re-implement TGraph::Chisquare as TH1::Chisquare using the ROOT::Fit::Chi2FCN class.
Add also option "R" to compute the chi2 in the given  function range (by default the range now is given by the data object - graph or histogram as it is done in Fit)