Support directed local clustering coefficient

[szarnyasg]

I am working to implement the LDBC Graphalytics workload using the Graph algorithms library. This requires implementing the directed variant of the local clustering coefficient (LCC).

The specification defines the LCC graph metric similarly to the definition in Wikipedia:

The important thing to consider is that edges that go directions are only considered between the neighbours, the neighbours of a given node are considered without taking directions into account.

Example graph, based on the lcc/dir-input and the lcc/dir-output files:

A smaller subgraph:

It's worth noting that most graphs in the basic Graphalytics data set are undirected with the exception of the following ones:

config-template/graphs$ ag 'directed = true'
twitter_mpi.properties
13:graph.twitter_mpi.directed = true

wiki-Talk.properties
13:graph.wiki-Talk.directed = true

example-directed.properties
12:graph.example-directed.directed = true

cit-Patents.properties
13:graph.cit-Patents.directed = true