Jw/examples

[eejwa]

PR notes

This pull request implements the following:

• Add a function to download the downsampled model.
• Change the examples to use the example model.
• Change examples to use the markdown format so they can used on the website docs.
• Moves the function to create a model for the convert example.

Pull Request Checklist. Please read and check each box with an X. Delete any part not applicable.

Describe what you did in this PR and why you did it.

Before your first pull request:

• [x] I have read the guidelines in our CONTRIBUTING.md document.

For all pull requests:

• [x] I have run the contrib/utilities/indent script from the main directory to indent my code.

For new features/models or changes of existing features:

• [x] I have tested my new feature locally to ensure it is correct.
• [x] I have created a testcase for the new feature/benchmark in the tests/ directory.

[anowacki]

Looks like the docs build script needs to install cartopy for the example script to get built.

[anowacki]

Also in the docs build logs:

INFO    -  The following pages exist in the docs directory, but are not included in the "nav" configuration:
  - SeismicLookupTable.md
  - copy_ref_examples.py
  - gen_ref_pages.py

This means you need to add the page to mkdocs.yml file under the nav section for the lookup table page to appear on the website.

[eejwa]

Okay no problem, thanks for the help Andy!

[anowacki]

@eejwa feel free to merge if you're happy. All looks good.

[anowacki]

Closes #128.

[eejwa]

Brilliant, thank you for the help Andy!