ducc0 for noise.py and other applications of FFT

[paganol]

Should we switch to ducc0 in noise.py? See, in particular, here and here

[mreineck]

Depends on the flexibility we need in the noise. If the documented 1/(f**alpha) spectrum with given f_knee and f_min is fine, then this is probably the fastest and most memory-saving approach, and it can produce infinite streams without discontinuities.
If we need more general noise spectra, the infrastructure from toast may be a better fit.

[paganol]

For now, I would go for ducc0 implementation. Should be easy to do, @martamonelli and @pcampeti already have experience with it, and they can work on this.