Nonuniform fast Fourier transforms

The NUFFTs are implemented thanks to Alex Townsend:

• nufft1 assumes uniform samples and noninteger frequencies;
• nufft2 assumes nonuniform samples and integer frequencies;
• nufft3 ( = nufft) assumes nonuniform samples and noninteger frequencies;
• inufft1 inverts an nufft1; and,
• inufft2 inverts an nufft2.

Here is an example:

julia> using FastTransforms

julia> n = 10^4;

julia> c = complex(rand(n));

julia> ω = collect(0:n-1) + rand(n);

julia> nufft1(c, ω, eps());

julia> p1 = plan_nufft1(ω, eps());

julia> @time p1*c;
  0.002383 seconds (6 allocations: 156.484 KiB)