Add materials for JupyterLab tutorial

[eyrei123]

Collection of notebooks, markdown and other files for Using JupyterLab tutorial.

Where to put new files:

• New files should go into a top-level subfolder, named after the article slug. For example: my-awesome-article

How to merge your changes:

1. Make sure the CI code style tests all pass (+ run the automatic code formatter if necessary).
2. Find an RP Team member on Slack and ask them to review & approve your PR.
3. Once the PR has one positive ("approved") review, GitHub lets you merge the PR.
4. 🎉

[martin-martin]

@eyrei123 the issues that you highlighted on Trello come from running black on the code in your notebooks.

If you take all the code in your notebooks, make sure that they're black-formatted, and that there aren't any superfluous empty cells at the bottom of the notebooks, then the checks should pass.

I demonstrated this with one of the files, Population Changes.ipynb, which now doesn't get flagged anymore.

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Please also move all your files into a dedicated folder, and also move your README.md file there. Currently, everything is in the root folder.

[martin-martin]

Closed in favor of #449