MNMG Leiden - Implementation

[ChuckHastings]

Implement Leiden algorithm.

Implementation should use the graph primitives. Because of Leiden's similarity to Louvain, it is expected that the existing primitives should be sufficient to implement Leiden. If there are new primitives that are identified they should be discussed and proposed as new issues so we can evaluate and prioritize.

Implementation should support SG and MG capabilities, although much of that is free by using the primitives.

Tuning will be done as a follow-on activity if necessary.