Add NEON support to FFTW - draft

We started working through the existing code in the simd directory of fftw, and here is what we learned about simd-sse.h. Our comments are in black serif text. We got about halfway through before other chores interrupted.

http://www.delmarnorth.com/microwave/software/simd-sse.h.discussion.draft.pdf

The plan here is to understand what the current set of simd code is doing, and then write code for neon to present for review. Several simd processors are already supported by FFTW, so learning how they work will help in creating code for NEON.

If you would like to participate, grab the latest version of FFTW source code (we're working with 3.2.2), go to the simd directory, and walk through the code. If you are already a black belt in FFTW simd code, then by all means speak up - we'd love to accelerate the process! We'll keep working on simd-sse.h until we understand it, then will move on to another header file. When all the header files are understood, we'll write down what we think a neon header file must accomplish.

-Michelle W5NYV