Update convolve2d to be exactly the flattened Simon's notebook version and...

Update convolve2d to be exactly the flattened Simon's notebook version and adjust fft2 consistently + add Simon's check_op function to validate a LazyLinearOp.

The previous version was violating linear operator properties.

Source: https://gitlab.inria.fr/delamare/faust_xp/-/blob/main/nb/2d.ipynb