feat: Graph.Nearest_Neighbor_Graph()

[szhorvat]

@Zepeacedust, this is an example of exposing a new graph generator in python-igraph, that you can later use as a model to expose the Delaunay generator.

This is not my comfort zone so let's wait for comments from @ntamas.

@ntamas, do you prefer Nearest_Neighbor_Graph or Nearest_Neighbor here? Since this term is used in other contexts as well, I vote for keeping the Graph suffix.

[Copilot]

Pull Request Overview

This PR introduces a new graph generator function Graph.Nearest_Neighbor_Graph() to python-igraph, which creates k-nearest neighbor graphs from spatial point data. The implementation serves as an example for exposing additional graph generators, specifically to guide the future implementation of a Delaunay generator.

Key changes:

• Adds C implementation of the nearest neighbor graph generator with support for different metrics (euclidean, manhattan) and configurable parameters
• Includes Python binding and method registration for the new graph generator
• Adds basic test coverage for the new functionality

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/_igraph/graphobject.c	Implements the C function and registers the Python method for nearest neighbor graph generation
src/_igraph/convert.h	Declares the metric conversion function for parameter parsing
src/_igraph/convert.c	Implements conversion from Python objects to igraph metric types
tests/test_generators.py	Adds basic test case for the new graph generator

Comments suppressed due to low confidence (2)

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.
You can also share your feedback on Copilot code review for a chance to win a $100 gift card. Take the survey.}