Add possibility to set model function (and also data) when using Fitter::FitFCN.

This gives possibility to call FitResult::GetConfidenceIntervals for the fits done using Fitter::FitFCN. For example in case of TBinomialEfficiencyFitter. 
This commit fixes ROOT-7790.
The tutorial TesBinomial.C has been updated to compute as example the confidence interval of the fitted function.