Mechanisms for graph transformation. These currently include:

- `DirectionTransformer`: generates graphs where input undirected edges have been converted to directed edges, or vice versa
- `FoldingTransformer`: transforms k-partite graphs or hypergraphs into unipartite graphs
- `VertexPartitionCollapser`: transforms a graph, given a partition of its vertices into disjoint sets, into a graph in which each of these disjoint sets has been 'collapsed' into a single new vertex.