pub fn idft(spectrum: &[Cplx]) -> Vec<Cplx> ⓘ
Inverse DFT: x[n] = (1/N) Σ_k X[k] · e^{+2πi kn/N}.
x[n] = (1/N) Σ_k X[k] · e^{+2πi kn/N}