(Graphviz) Nodes don't show cache-specific information

WriteDotFile uses n->serialize(false, "") in order to get a Node's metadata. This excludes metadata elements which are only included when caching is enabled (e.g. cached file's name).

I currently can't think of a good way to fix this without introducing more complexity. Options I can currently think of:

• Honor Graph::cacheEnabled: Dot file will include info, but files in the cache will be rewritten.
• Separate serialize_ into serialize_meta_ and serialize_data_: WriteDotFile then just calls n->meta(cacheEnabled()) or something. More control over what gets written when, but also more code for a Node implementer to maintain. It also makes it difficult/impossible to conditionally write metadata based on the success of serialize_data_. Might be doable alongside #10 (closed).
• Rework Graph so that its most recent Metadata is always available: WriteDotFile can then use this to get the set of serializable properties. I think this might have the fewest issues of the current ideas, but hits the issue that Serialize honors cacheEnabled(). What should Graph::metadata() produce if update hasn't been called caching is enabled? We don't really want to serialize to disk in that circumstance.