Zara Zong

Results 2 issues of Zara Zong

New PR for [rich-club coefficient](https://github.com/igraph/igraph/issues/2726) --- branching off `master` instead of `develop`. Currently working on vertex removal discussed [here](https://github.com/igraph/igraph/pull/2738#issuecomment-2752616899). - [x] By submitting this pull request, I assign the copyright...

- [x] By submitting this pull request, I assign the copyright of my contribution to _The igraph development team_.