ACF w/o SIMD

[headupinclouds]

We have GPU and CPU options for ACF computation. The shader based GPU implementation may need more work to achieve desired accuracy. For the CPU path there is NEON and SSE support (thanks to NEONvsSSE.h header). Some of this code will currently no run on the CPU without SIMD. Boost.SIMD might be worth a look to generalize some of that code.