Back out pull request 6150, which is incompatible with ROOT6

This is just a request to revert #6150, which uses deprecated TMinuit functionality that is no longer supported in ROOT6.
TFitterMinuit is an obsoleted interface in ROOT 6. To minimize a 
function using Minuit2, one should use the ROOT::Math::Minimizer 
interface or directly ROOT::Minuit2::Minuit2Minimizer
see 
http://root.cern.ch/drupal/content/numerical-minimization#multidim_minim
This is a large change, so code commonality should be maintained.
Please revert 6150 and redo it using the one of the two supported interfaces, as documented in the link above.