find_shift should return 2 values + poly fit
In order to implement the longitudinal alignment procedure "alignxc", it is needed that the function find_shift returns max correlation also in the vertical plane. Usually the V value is near 0 for center of rotation calculation but not in the alignxc case. Problem: the polynomial fit to reach sub-pixel precision works without padding (the values are periodic in Fourier space) but I wonder what will happen with padding.