Complete the R implementations of usability benchmarks

[szhorvat]

We have a set of usability benchmarks—a collection of small tasks that help us judge how convenient it is to use the library. This issue is to implement each such task using R/igraph.

https://github.com/igraph/usability-benchmarks

This collection of solutions will also be useful as a "dictionary" that helps users familiar with another high-level interface of igraph (or even a different graph library) get up to speed with R/igraph.

If you plan to work on this issue, choose one or a few exercises of your liking, and post a comment noting which ones you will implementing.