matrices: add dual attribute

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 83.23%. Comparing base (7e7b662) to head (8dac32a).

Files	Patch %	Lines
linopy/matrices.py	60.00%	2 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #258 +/- ## ========================================== - Coverage 89.02% 83.23% -5.79% ========================================== Files 15 15 Lines 3635 3645 +10 Branches 852 653 -199 ========================================== - Hits 3236 3034 -202 - Misses 277 489 +212 Partials 122 122 ```

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