replacing isnan/isfinite by std::isnan/std::isfinite

Here I replaced isnan and isfinite function calls by std::isnan and std::isfinite, because it is not guarannteed by the C++ standard that the compiler brings these functions to the global namespace when including the c++ header (see https://stackoverflow.com/questions/18128899/is-isnan-in-the-std-namespace-more-in-general-when-is-std-necessary-optio).

I faced some compile issues with this today. I think this changes should not affect the functionality.