center indices of `fftshift`

I thought it might make sense to have a function, e.g., `centerIndex`, returning a tuple of indices that points to the center of a grid after an `fftshift`.
In other words, instead of spreading `div.(size(x),2)` here and there, extracting it into a separate function.
So, by calling it, one can avoid warily checking their shifts/offsets are consistent with what's been used in the `fftshift`.

I can make a PR if this is of interest.