Merged requested to merge RichFuture-VectoriseRayTracing into master
Relies on LHCb!565 (merged)
Uses CPU capabilities detection at run time to select SSE4/AVX/AVX2+FMA implementations.
AVX2+FMA currently disabled (AVX used instead) until such a time as all build targets support it (requires an updated binutils for SLC6). Decided to also disable this for centos7, even though there it could be built, in order to not introduce potential platform differences.