Anisotropic Diffusion

Revisions to the lab on Anisotropic diffusion.

- [x] Revise formatting.
- [x] Add example code.
- [x] Comment in example code.
- [x] Add additional explanation of how the optimizations should work.
- [x] Rewrite second problem with hints based on example.
- [x] Remove first problem.
- [x] Fix alignment of equations so they stay within the margins.
- [x] Briefly remind them how to read and write images.
- [x] Add hints and possibly examples for how to broadcast along a 3D array to filter a color image.
- [ ] Give a short explanation of what numexpr is and how to use it.
- [ ] Modify code examples so that they are more similar to one another.
- [ ] Verify that naming conventions are consistent across examples.
- [ ] Check that lambdas are present in all places where they should be.
- [ ] Add comments to the slow code examples.
- [ ] In the derivation, emphasize where the modification happens (and that it isn't an equality).
- [ ] Talk them through the optimizations that reduce the repetitive computations.