Improve the Tutorial for using relax.methods

[BirkhoffG]

We have an incomplete tutorial on using relax.methods. Let's start to improve this tutorial.

Delivery

1. git checkout tutorial
2. Go to tutorials/methods.ipynb
3. Run a grammar check on the entire notebook to correct any spelling/grammar mistakes.
4. Finish all four TODOs.
5. Create a pull request to commit your changes. Please also read this before creating a pull request.

Rules-of-Thumb in Writing Tutorials

The tutorial should be accessible to developers. Use as many examples as possible so that the developers can just copy/paste your code for their usage.