- add a new function TMath::Power(double, int) impelmented using std::pow(double,int) since it is 100% faster

- fix for new Power function some code in TMVA 

- Fix in TMath implementation of IsNaN for the latest C++ compiler when used together with cmath 
  see https://savannah.cern.ch/bugs/?87534


git-svn-id: http://root.cern.ch/svn/root/trunk@41453 27541ba8-7e3a-0410-8455-c3a389f83636