FStream: Make (i|o)fstream::is_open function public

It is public in std. See: http://en.cppreference.com/w/cpp/io/basic_ifstream/is_open

This function being private causes this to fail: https://open.cdash.org/index.php?compare1=61&filtercount=1&field1=revision&project=CMake&showfilters=0&limit=100&value1=acedd697f480f4cbe58b751259ff32de2ff24da6&showfeed=0