FFT: Support arbitrary sized vectors using Bluestein's algorithm

[gsgou]

could be based on mathnet-numerics implementation:
https://github.com/mathnet/mathnet-numerics/blob/master/src/Numerics/Providers/FourierTransform/ManagedFourierTransformProvider.Bluestein.cs#L252

[swharden]

... okay this may be pretty easy actually. I found this, and this could be as easy as copy/pasting it in. It looks to be under a MIT license:

https://www.nayuki.io/res/free-small-fft-in-multiple-languages/Fft.cs

[swharden]

This needs more work so I'm leaving it in the Experimental namespace so I can publish the package right now, but we can test this a little more thoroughly and add it into the main API soon. I'm merging #80 in so we both have access to the code if/when one of us decides to pick it up!

// this will work when I publish the new package in a few minutes
double[] prime = { 1, 2, 3, 4, 5, 6, 7 };
System.Numerics.Complex[] result = FftSharp.Experimental.Bluestein(prime);

[gsgou]

tks a lot @swharden, we can catch up on this at a later time.