updated TopPtReweight to follow recommendation

updating the code to the official prescription for top pt reweighting [1], which is valid only for pt<400
if pt>400, the scale factor is calculated for pt=400 (same as the all-hadronic zprime analysis)

[1] https://twiki.cern.ch/twiki/bin/viewauth/CMS/TopPtReweighting#MC_SFs_Reweighting