Switch to std::thread for loading the RM volumes
Switch to std::thread for loading the RM volumes

This provides better portability across win/lin/osx

wilsonCernWq/ospray,MengjiaoH/ospray,wilsonCernWq/ospray,ospray/OSPRay,ospray/OSPRay,wilsonCernWq/ospray,ospray/OSPRay,MengjiaoH/ospray,MengjiaoH/ospray,MengjiaoH/ospray,MengjiaoH/ospray,ospray/OSPRay