Portability issues and some fixes

[boristim66]

Dear Rajendra Thanks for the very interesting utility. Good news. Build with Gromacs-2019 (4 )/ Windows10 VisualStudio2019 successfully passed and works. But I had to fix some portability issues and errors - if the xtc file contains only DNA/RNA (solvent and ions excluded), the function reset_x causes exception because out of range on the array. In addition, I had to put additional protection - sometimes there are system races ?/caching? when opening/closing a file with find_pairs. If this is interesting, I attach a patch for further development of the project. I will test on versions of gromacs 2020 and 2021. RAMDisk required. Best regards, Boris. diff.txt