add examples.py

[FabianHofmann]

• A new module examples was added which contains example models. For example, you can call m = linopy.examples.benchmark_model().

[codecov[bot]]

Codecov Report

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

Project coverage is 84.40%. Comparing base (d7a4d74) to head (863a43a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #286 +/- ## ========================================== - Coverage 88.97% 84.40% -4.58% ========================================== Files 15 16 +1 Lines 3711 3744 +33 Branches 876 877 +1 ========================================== - Hits 3302 3160 -142 - Misses 283 464 +181 + Partials 126 120 -6 ```

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