Add load tests

[MarcSkovMadsen]

This PR builds on #11 and #26. Review those PRs first!

Works around #30

In order to keep the code best practice and recommendable we need to make sure the apps can load fast.

This will also make it much easier for us for example to deploy them to Hugging Face spaces as requested in #9.

This PR adds load testing using Locust. See the updated README for details.

[MarcSkovMadsen]

I've merged with the current main branch and

• added missing dependencies
• cleaned up issues
• fixed tests
• separated developer instructions in DEVELOPER_GUIDE.md and made it work end to end. Closes Please separate user and developer instructions #28
• added the short cut command hatch run panel-serve. Closes Please add short cut command #23

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

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Additional details and impacted files

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