[igraph] Suggestion: Include cluster comparison functions.

[Gang Su]

Dear Developers,

I have a suggestion. As different algorithms have different community 
detection results, you may think about incorporating some measure for 
comparing two partitions.

One suggestion is adjusted Rand Index, which has been implemented in 
package mclust, adjustedRandIndex()

There are other indexes, such as J index, rand index
A reference is attached here:
http://faculty.washington.edu/kayee/pca/supp.pdf

Gang