#711 Traversal v1 (4/4): Python client methods and traverse.md

Last of four stacked PRs for labeled traversal, on #710.

This PR adds GraphClient.resolve() and GraphClient.traverse() to the Python client, and metagraph/docs/traverse.md. The document covers the routes and the CLI, the request and response schemas, the walk rule, the end reasons, the bounds and what version 1 leaves out of gr/labeled-traversal. An integration test runs the client against a server.

No caller uses the client methods yet: the staging service calls the HTTP routes directly. They give Python users the same access to the experimental routes that search() and align() give to the others.

Co-authored-by: Gunnar Rätsch

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