iteration function for fourier averaging

[hcwinsemius]

Allow for longer videos to be sampled, with several slices in time.

Possible function signature:

def sliding_window_spectrum(
    imgs,
    win_t=100,
    overlap=50,
    **kwargs
):
    # computation of spectra as many times as fits in the amount of frames and overlaps, using `spectral.spectral_imgs`.
    **kwargs are passed to the `spectral_imgs` function