Data structure traversal support

[hunterhector]

NLP data structures themselves are often interesting. It is often required to traverse a tree or follow a path in a graph in NLP. For example, one can follow the dependency tree to find the headword, or follow a network path (following specific relation links). This can be supported by Forte in a natural way, and we can at the same time attach relevant features to the path (e.g. embeddings along the path, or relation labels along the path).