Support pickling a linopy model

[victorgarcia98]

Closes #340 .

Changes proposed in this Pull Request

Defining __getstate__ and __setstate__ helps avoid a potential recursion error during pickling, especially in classes that override __getattr__. Since __getattr__ is called whenever an attribute is not found, an implicit or default pickling process might unintentionally trigger it, leading to infinite recursion. By explicitly specifying how the object's state should be serialized (__getstate__) and restored (__setstate__), the class ensures that only its actual attributes are processed, preventing unnecessary lookups and recursion errors during pickling and unpickling.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[FabianHofmann]

Wonderful @victorgarcia98! Thank you for your contribution!