A community detection algorithm: Leiden with `PIE` model

Implement a community detection algorithm: Leiden with PIE model.

Paper: https://arxiv.org/abs/1810.08473

Reference implementation:
- https://cdlib.readthedocs.io/en/latest/reference/cd_algorithms/algs/cdlib.algorithms.leiden.html#cdlib.algorithms.leiden
- https://github.com/vtraag/leidenalg

Start by reading the linked paper and the CDlib and leidenalg reference implementations; the issue names no repository files, entry points, or tests. Determine where community-detection algorithms are integrated, then implement Leiden with the PIE model and add or run coverage demonstrating the algorithm works.

Written by the indexing model from the issue text.