Return an slice with a symmetric region around center.

def slice_create(center, block, start=0, stop=None):
    '''Return an slice with a symmetric region around center.'''

    do = coor_to_pix_1d(center - block)
    up = coor_to_pix_1d(center + block)

    l = max(start, do)

    if stop is not None:
        h = min(up + 1, stop)
    else:
        h = up + 1

    return slice(l, h, 1)