DocumentFactory and the variable mesh datasets

[matthewturk]

Jupyterlab provides an extremely rich set of methods for interacting with and providing document information. This includes having documents be registered and displayed in a persistent manner.

This is something we should certainly explore for widgyts. It would require implementing a document factory, a document widget, and a model, but this is possible with our existing widgets, etc.

[matthewturk]

I've started exploring some of this with #73 . I think the better match might be the @jupyterlab/running and @jupyterlab/running-extension` packages.