Chore: Rephrasing of atomic_system.py docstring

[97gamjak]

I just adjusted the test case for str in the cell class, which was failing due to a new numpy release.

[codecov[bot]]

Codecov Report

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

Project coverage is 85.43%. Comparing base (099b132) to head (63baa93). Report is 1 commits behind head on dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #106 +/- ## ======================================= Coverage 85.43% 85.43% ======================================= Files 125 125 Lines 5027 5027 ======================================= Hits 4295 4295 Misses 732 732 ``` | [Flag](https://app.codecov.io/gh/MolarVerse/PQAnalysis/pull/106/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=MolarVerse) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/MolarVerse/PQAnalysis/pull/106/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=MolarVerse) | `85.43% <100.00%> (ø)` | | | [Files](https://app.codecov.io/gh/MolarVerse/PQAnalysis/pull/106?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=MolarVerse) | Coverage Δ | | |---|---|---| | [PQAnalysis/atomic\_system/atomic\_system.py](https://app.codecov.io/gh/MolarVerse/PQAnalysis/pull/106?src=pr&el=tree&filepath=PQAnalysis%2Fatomic_system%2Fatomic_system.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=MolarVerse#diff-UFFBbmFseXNpcy9hdG9taWNfc3lzdGVtL2F0b21pY19zeXN0ZW0ucHk=) | `97.61% <ø> (ø)` | | | [PQAnalysis/core/cell/cell.py](https://app.codecov.io/gh/MolarVerse/PQAnalysis/pull/106?src=pr&el=tree&filepath=PQAnalysis%2Fcore%2Fcell%2Fcell.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=MolarVerse#diff-UFFBbmFseXNpcy9jb3JlL2NlbGwvY2VsbC5weQ==) | `100.00% <100.00%> (ø)` | |