refactor: Simplify yearbook data generation

[robinholzi]

Motivation

As yearbook is a rather small dataset, I propose to always store the train, test and all (new) variants.

Changes

• Always output all three splits of the datasets
• Use pathlib instead of os.path for cleaner code

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.34%. Comparing base (beb3769) to head (fee28d5).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #523 +/- ## ======================================= Coverage 82.34% 82.34% ======================================= Files 216 216 Lines 10085 10085 ======================================= Hits 8304 8304 Misses 1781 1781 ```

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[github-actions[bot]]

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