check if h5 file exists at xml read

[walkloud]

make a little utility to add this file existence check

#include <unistd.h>

        bool file_exists( const std::string &filename )
        {
            return access( filename.c_str(), 0 ) == 0;
        }

so that we can check if the h5 file exists at the time of reading xml before doing any work.