Cleanup `DataContainer` in preparation for Effect refactoring

[teutoburg]

Made some attributes private, added some typing, added some comments, some general refactoring.

This will make it a bit easier and cleaner to do the planned changes to the Effect class.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.27273% with 7 lines in your changes missing coverage. Please review.

Project coverage is 76.58%. Comparing base (ba570af) to head (1eacb10). Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
scopesim/effects/data_container.py	85.10%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #478 +/- ## ========================================== - Coverage 76.61% 76.58% -0.03% ========================================== Files 66 66 Lines 8098 8094 -4 ========================================== - Hits 6204 6199 -5 - Misses 1894 1895 +1 ```

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