Cfg dataset; better assertions and comments

[arturtoshev]

• make dataset a group of config parameters
• improve usability with better assertions and comments

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.1%. Comparing base (6428cff) to head (9f1f397).

Files	Patch %	Lines
lagrangebench/data/data.py	50.0%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #31 +/- ## ===================================== Coverage 82.1% 82.1% ===================================== Files 25 25 Lines 1680 1686 +6 ===================================== + Hits 1380 1385 +5 - Misses 300 301 +1 ``` | [Flag](https://app.codecov.io/gh/tumaer/lagrangebench/pull/31/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tumaer) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/tumaer/lagrangebench/pull/31/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tumaer) | `82.1% <91.6%> (+<0.1%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tumaer#carryforward-flags-in-the-pull-request-comment) to find out more.

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[arturtoshev]

Does not disrupt anything, so fine by me. In the end you didn't change the logic of the dataset class. Should we speity in the docstring to the dataset that download starts if the directory does not exist?

  dataset_path: Path to the dataset. Download will start automatically if dataset_path does not exist.

there you go: 9f1f397