suggestion: replace np.roll by scipy.ndimage.fourier_shift

[alexlib]

similar to https://scikit-image.org/docs/stable/auto_examples/registration/plot_register_translation.html#sphx-glr-auto-examples-registration-plot-register-translation-py

it will allow us to use subpixel shifts and more stable and accurate tests

[ErichZimmer]

Where do we use this? Only on tests and the basic theory notebook?

[alexlib]

only in tests. tests is our weakest point

[ErichZimmer]

I suppose I can add more tests, probably in a similar fashion to OpenPIV-Python-cxx, while implementing your idea.