Robust serialization

[edparcell]

The current serialization method uses dill to serialize the entire graph, less some node properties that are known not to serialize well. Deserialization then requires access to the modules originally used to calculate a graph, and if methods or structures within those modules have changed in the meantime, the entire graph may be unrecoverable.

Instead, it would be better to serialize graph structure and each node independently, and optionally using multiple methods, so that even if a preferred method is not usable, the data itself is still accessible and recoverable.

One method to do this would be to have a zip file, with the graph structure represented by one file, and each node represented by one or more zip files.