Tools for working with FFT binning

[brainandforce]

I've created the new FFTBins and FFTLength types that can be used to work with data that's been Fourier transformed and types whose indices are effectively FFT bins.

[codecov[bot]]

Codecov Report

Patch coverage: 65.38% and project coverage change: +0.75 :tada:

Comparison is base (7a959cc) 39.01% compared to head (3b0ee9a) 39.77%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #154 +/- ## ========================================== + Coverage 39.01% 39.77% +0.75% ========================================== Files 16 16 Lines 1297 1315 +18 ========================================== + Hits 506 523 +17 - Misses 791 792 +1 ``` | [Impacted Files](https://codecov.io/gh/brainandforce/Electrum.jl/pull/154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brandon+Flores) | Coverage Δ | | |---|---|---| | [src/Electrum.jl](https://codecov.io/gh/brainandforce/Electrum.jl/pull/154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brandon+Flores#diff-c3JjL0VsZWN0cnVtLmps) | `0.00% <ø> (ø)` | | | [src/data/fft.jl](https://codecov.io/gh/brainandforce/Electrum.jl/pull/154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brandon+Flores#diff-c3JjL2RhdGEvZmZ0Lmps) | `28.57% <0.00%> (ø)` | | | [src/data/grids.jl](https://codecov.io/gh/brainandforce/Electrum.jl/pull/154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brandon+Flores#diff-c3JjL2RhdGEvZ3JpZHMuamw=) | `50.00% <0.00%> (+7.89%)` | :arrow_up: | | [src/math.jl](https://codecov.io/gh/brainandforce/Electrum.jl/pull/154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brandon+Flores#diff-c3JjL21hdGguamw=) | `88.57% <80.95%> (-11.43%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brandon+Flores). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brandon+Flores)

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