Reformat: get rid of Orekit-based API

[jorgepiloto]

After a private discussion, we decided to get rid of the custom Orekit-based API and make use of its pure Python wrapper. This new implementations includes the following features:

• The orekit/src/ has been completely removed as it is no longer required.
• Test files and functions are renamed validate_*.py. This outputs a cool namespace in the CI actions, from my point of view.
• Regarding the orekit-data.zip, it is ignore from the VCS no matter its location and will be downloaded if not found. This behaviour is coded under the tests/conftest.py.
• The Orekit's virtual machine needs to be executed in each one of the scripts. Otherwise, I have experienced segmentation-fault errors, strange non-traceable issues and non-sense numerical values.

(Edit: Supersedes #15)

[jorgepiloto]

I will squash commits and fix quality checks. Then, this will be ready to review :+1:

[astrojuanlu]

🎉 Thanks @jorgepiloto !