[TMVA] Use TMath versions of std::isinf and std::isnan
[TMVA] Use TMath versions of std::isinf and std::isnan

this is useful since the ROOT versions do identify infs and nans
also in "fst builds", i.e. with --fast-math also enabled.

gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root