center indices of fftshift

[tianrluo]

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.