[2d] Extend the python library to support declaring models

[alaric-dotmesh]

As a dotscience user, I'd like to be able to use the Python library to declare that my run has produced one or more models, so that I can then use CI integration features to publish those models to production.

See #7 for general support for declaring directories as outputs, which might be a useful first step.

ACs

• I can declare a file or directory as a named model: If I write code like ds.model("roadsigns", "./model"), the library will attach a label to this run of the form model:roadsigns={"files":{"main":"./model"},"metadata":{}}, and declare ./model as an output if it's a file, or recursively declare all of the files inside it as outputs if it's a directory.
• I can declare a map of names to files/directories as parts of a named model: If I write code like ds.model(“roadsigns”, {“main”: “./model”, “classes”: “./classes.csv”}), the library will attach a label of the form model:roadsigns={"files":{“main”:”./model”, “classes”: “./classes.csv”},"metadata":{}}, and likewise recursively declare ./model as an output.
• I can attach arbitrary key=value metadata to models: If I write code like ds.model("x","y", foo="bar"), the library will attach a label of the form model:x={"files":{"main":"y"}, "metadata":{"foo": "bar"}}, and declare y as an output.