Add CompiledModel class which can generate DataFrames and Tables

[gtfierro]

This changes Model.compile so it returns a new CompiledModel class. This is not stored in the database; it is an ephemeral class which stores the compiled model and provides some convenience methods. In particular, it can generate DataFrames or SQL tables from SHACL shapes.

See example here

@TShapinsky @MatthewSteen what do you think of the abstraction? Once we figure out what this should look like then I can propagate the changes to the rest of the codebase and add tests

[gtfierro]

From dev discussion:

• can we avoid duplicated code? Factor them out into their own "superclass"
• move this to its own file?
• keep it out of the database (stay in-memory)