expose rescale_rfft functions

[alisterburt]

over at teamtomo/membrain-seg#76 @LorenzLamm and @JoelVO like torch-fourier-rescale but wants to reweight frequencies prior to fourier pad/crop

We should expose the lower level, currently private _rescale_rfft_2d/3d functions

docs will be required for people to use this well - example below can serve as a template

proposed API:

import torch
import torch.fft
from torch_fourier_rescale import fourier_rescale_rfft_3d
from my_package import reweight

volume = torch.rand((32, 32, 32))

# fftshift so that origin of phases in DFT is in the center of the image
# this maintains the center of the image at the position of DC component pre/post rescaling
volume = torch.fft.fftshift(volume, dim=(-3, -2, -1))
dft = torch.fft.rfftn(image, dim=(-3, -2, -1))
dft = torch.fft.fftshift(dft, dim=(-3, -2))

# do the reweighting
dft = reweight(dft)
dft = rescale_rfft_3d(dft)

# transform back to real space and recenter
dft = torch.fft.ifftshift(dft, dim=(-3, -2))
rescaled_volume = torch.fft.irfftn(dft, dim=(-3, -2, -1))
rescaled_volume = torch.fft.ifftshift(rescaled_image, dim=(-3, -2, -1))