Faster generate_matrix function

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.94%. Comparing base (e67fd4e) to head (ac680cc). Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
openparticle/utils.py	0.00%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #41 +/- ## ========================================== - Coverage 80.40% 79.94% -0.47% ========================================== Files 8 8 Lines 1337 1730 +393 ========================================== + Hits 1075 1383 +308 - Misses 262 347 +85 ```

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