Refactor and GPU tests

[marvinfriede]

• add --cuda flag for running all tests in GPU
• improve triples mask
• restructure to be consistent with tad-dftd3

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (82a9ee1) 98.84% compared to head (da33041) 97.56%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/tad_dftd4/utils/grad.py	40.00%	9 Missing ⚠️
src/tad_dftd4/charges.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
- Coverage   98.84%   97.56%   -1.28%     
==========================================
  Files          23       26       +3     
  Lines         519      576      +57     
  Branches       49       55       +6     
==========================================
+ Hits          513      562      +49     
- Misses          3       12       +9     
+ Partials        3        2       -1     

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