fn block_internal_lengths(&self) -> [usize; 3] {
        [
            self.options.orientations,
            self.options.block_side,
            self.options.block_side,
        ]
    }
Dimensions of a single block descriptor, viewed as a 3d array.
Innermost dimension is histogram bin, then horizontal cell location, then
vertical cell location.
src/hog.rs