Anisotropic Diffusion

Revisions to the lab on Anisotropic diffusion.

- [ ] Revise formatting.
- [ ] Add example code.
- [ ] Comment in example code.
- [ ] Rewrite second problem with hints based on example.
- [ ] Remove first problem.
- [ ] Briefly remind them how to read and write images.
- [ ] Add hints and possibly examples for how to broadcast along a 3D array to filter a color image.