Non-deterministic CDLP algorithm

[szarnyasg]

I had an interesting discussion with the igraph authors at their forum. They highlighted that the non-deterministic CDLP algorithm isn't only faster but also better in some cases (as it tends to avoid the oscillation of labels). So when touching the CLDP codebase, I'll also add a boolean flag to set deterministic/non-deterministic behaviour.

[szarnyasg]

A correction to my earlier post: the nondeterminism (always picking the smallest label) is not the main cause of the oscillation - instead, the problem is the synchronous nature of the algorithm. An asynchronous algorithm does better for avoiding the problem.

A potential workaround is partitioning the work, then

1. select k vertices and their incoming edges in the matrix randomly
2. compute the new labels
3. assign the labels to vector l
4. if all vertices were processed, we're done, otherwise go to 1.