Added the Leiden algorithm#399

Merged

gaborcsardi merged 2 commits intoigraph:devfrom

vtraag:feature/leiden

Oct 23, 2020

Commits on Jun 26, 2020

Added cluster_leiden.
vtraag
committed

Commits on Jun 27, 2020

cluster_leiden: corrected documentation. Also removed gaborcsardi/roxygen from required packages in Travis.
vtraag
committed