Reorganize tests

[tbittar]

This PR reorganizes tests as follows:

• Creates and e2e directory to store all end-to-end tests
• Move some tests that were not within a relevant file / directory to a more relveant one
• Move librairies, systems, timeseries data to subfolders closer to the tests that actually used them
• Add docs on some end-to-end test files

Some refactoring is to be made to clean some library files, this will not be done in this PR

[sylvlecl]

Nice, it looks really better