std::filesystem Usage Improvement, master branch (2019.10.16.)

In certain situations it is necessary to overrule the decision of the Gaudi code on whether std::filesystem is safe to use or not. For such cases there is now a cache variable (GAUDI_USE_STD_FILESYSTEM) in place, that one can use from the outside to control this behaviour.

I ran into this while testing a non-standard, LLVM based compiler that the current Gaudi configuration decided was appropriate for using std::filesystem with. But as it turned out, it wasn't...