WIP: Remove VecCore from VecGeom and use external project
This merge request removes the old bundled VecCore from VecGeom and adds it back as an external project.
The added benefit is that with -DBUILTIN_VECCORE=ON
VecGeom can now use VecCore to download and install Vc and/or UME::SIMD.
It also simplifies how to configure VecGeom a little bit, but this is work in progress.
Note: On my computer CUDA compilation is broken (master branch, using CMake 3.7.2, not this branch). It seems that CUDA cannot find the VecGeom headers.
Not sure if it's a configuration problem on my side or not. Please test this branch and let me know if you find any problems.
So far I can compile both with pre-installed VecCore and built-in compilation of VecCore, for both Vc and UME::SIMD backends, using GCC 5.3.0.