add pytest for the write_cube method in Cp2kCube class

[codecov[bot]]

Codecov Report

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

Project coverage is 47.25%. Comparing base (26e2df3) to head (19e4202). Report is 25 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #70 +/- ## ========================================== + Coverage 46.33% 47.25% +0.92% ========================================== Files 32 32 Lines 1951 1951 ========================================== + Hits 904 922 +18 + Misses 1047 1029 -18 ```

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