Pull request type

• ReadMe, Docs and GitHub updates

Checklist

• [x ] Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally
• CHANGELOG.md has been updated (if relevant)

Current behavior

The data and tests folder are really messy, see issue #281

New behavior

• Reorganized the data folder
• Avoided file repetitions
• Updated paths in the tests and examples notebooks
• Both slow and normal tests are passing locally.
• Still needs to build the documentation on readthedocs and verify it is working.

Breaking change

• No, but some examples may fail due to "file not found error", so we need to keep an eye on that.

Additional information

At the beggining it seemed an easy task, but believe me it wasn't.