Write a direct implementation of betweenness for testing purposes, to verify the main implementation. This will be based on direct enumeration of all shortest paths between vertex pairs. Ideally it should support a cutoff distance as well as using a subset of source/target vertices.

This function would be part of a unit test, as it's meant to be used for verifying other betweenness implementations.

See igraph_get_all_shortest_paths and igraph_get_all_shortest_paths_dijkstra.