Add image pyramid structure

[jonysy]

The current pyramidal implementation of the Lucas-Kanade tracker doesn't accept image pyramids. Implementing Flow<Pyramid> for PyramLucasKanade would allow for precomputed image pyramids.

[pub] [type | struct] ImagePyramid<I = GrayImage> [= Vec<I>; | { images: Vec<I>, orientation: .. }]

impl Flow<ImagePyramid> for PyramLucasKanade { .. }