FFT Lib?

[dsimcha]

== Quote from Fawzi Mohamed (fawzi at gmx.ch)'s article
> I was preceded, that is a good list. I can only say that FFTW is
> *really* good, and if you structure your code well you should be able
> to support more than one library.
> I have wrappers for fftw, and it can be used with NArray (N
> dimensional dense arrays in blip), but is not directly there exactly
> due to the license.
> So having a fallback FFT would be useful for me.
> ciao
> Fawzi

This was kind of my point:  Does a "simple and good enough" FFT that's O(N log N)
but not super optimized and maybe works on generic ranges and ranges of ranges
belong in Phobos?  The idea is that if you just need a few FFTs here and there,
convenience is important, and they're not a major bottleneck for you, you use
Phobos.  If you need the absolute best even if it has to be installed separately,
is under a more restrictive license, is more of a PITA to use, etc., you use FFTW
or something.

My use case, to give a little more detail as an example, is that I want to add
kernel density estimation to dstats.  I need a reasonably efficient way to compute
a convolution, but not super-optimized pedal-to-the-metal ones, and I need to not
have to add a dependency to dstats just for this and for the license to be compatible.