Added the Leiden algorithm

[vtraag]

I've added the Leiden algorithm to the R interface. I've also added a simple test.

The functions.def did not yet include igraph_community_leiden in the igraph C core. For that reason the cigraph submodule points to this commit in the fix/leiden-R branch for now. If all is well here on the R side as well, we can merge fix/leiden-R to the master branch, and update the cigraph submodule to the master branch.

[vtraag]

I just realised I forgot to update the documentation, it was just copy-pasted from Python interface.

[vtraag]

This PR closes #346 by the way.

[ntamas]

Merged igraph/igraph#1422 now; I leave it up to someone familiar with the R internals to merge / close this one.

[timoast]

Hi, just wondering if there is any rough timeline for this being merged? We're very interested in using this functionality in the Seurat R package once it's available. I'd be happy to help if there's anything I can do to move this along

[vtraag]

We should ask @gaborcsardi. As far as I am concerned this is good to go.

[TomKellyGenetics]

@timoast I will update the leiden R package to call this (as it should improve performance) when it becomes available. Of course, you are also welcome to change Seurat, let me know if you want help with it.

[gaborcsardi]

Thanks!