Heatflow

Edits to the heat flow lab.

- [ ] Add a diagram to explan how the updating process works.
- [ ] Add another diagram to show how the Crank-Nicolson method updates.
- [ ] Re-explain the tridiagonal solver more thoroughly.
- [ ] Show matrix version of differencing schemes that does account for boundary conditions.
- [ ] Add a warning box about the scoping issues that sometimes arise in the animation function.