Filtering mechanisms that produce subgraphs of an original graph. Currently includes:

- `Filter`: an interface for graph filters
- `{Edge,Vertex}PredicateFilter`: graph filters that return the induced subgraph according to the specified edge or vertex `Predicate`, respectively.
- `KNeighborhoodFilter`: a filter that returns the subgraph induced by vertices within (unweighted) distance k of a specified vertex.