Add support for leiden algorithm

[robertamezquita]

https://cran.r-project.org/web/packages/leiden/vignettes/run_leiden.html - would be great to add a native igraph method for this, seems like a fair amount of the legwork has been done.

[vtraag]

Thanks for the request! It's on my to-do list (I'm one of the authors of the Leiden algorithm).

[SamGG]

Hi Vincent,
Any idea when leiden_community will be available in rigraph?
Best

[vtraag]

We first have to upgrade to the new C core, after which I can start to include other new contributions from that new C core, among which, the leiden algorithm. This is done in PR #380, we are awaiting comments / merging by @gaborcsardi.

[SamGG]

I hope @gaborcsardi will find time soon. There are a lot of impatient programmers as me ;-)
Thanks.

[ntamas]

The Leiden algorithm is now available in igraph 1.2.7 as cluster_leiden().