Add partition weighting (#106)

* Add partition weighting

* Update pymetis/__init__.py

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* Update pymetis/__init__.py

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* Update pymetis/__init__.py

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* Style: fix formatting errors flagged by Ruff

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Co-authored-by: Andreas Klöckner <inform@tiker.net>