long which to pad `arr`.
width_pair : (int, int)
    Pair of widths that mark the pad area on both sides in the given
    dimension.
original_period : int
    Original length of data on `axis` of `arr`.

Returns
-------
pad_amt : tuple of ints, length 2
    New index positions of padding to do along the `axis`. If these are
    both 0, padding is done in this dimension.
r