refactor(settings): refactor settings to create an user oriented class

[MartinBelthle]

What's done this PR achieves:

• Introduces a dataclass StudySettings for the user
• Use this class inside the Study object (before it was the union of 2 types local and API)
• Implement the method update_settings for local
• Uses the new endpoint of AntaresWeb (not merged yet 😄 ) so the code won't work until next web release (it's already the case due to the read_links method)
• Removes test for playlist and thematic trimming (inside local) as they didn't handle real cases with multiple keys

What should be done next:

• Introduce multiple keys ini reader/writer for thematic trimming and playlist (for local)
• Handle playlist and thematic trimming reading and writing (for local)
• Add tests (for local)

NB: I forgot what we said Sylvain about the splitting of the files study_settings_api and local between models and methods