fn num_blocks(num_cells: usize, block_side: usize, block_stride: usize) -> usize {
    (num_cells + block_stride - block_side) / block_stride
}
Number of blocks required to cover `num_cells` cells when each block is
`block_side` long and blocks are staggered by `block_stride`. Assumes that
options are compatible.
src/hog.rs