Re-implementing framework.py and model.py's API on top of Pydantic

[JGoldstone]

I've written a not-quite-done-but-pretty-far-along re-implementation of the Clip class, and the various user-visible classes it uses, on top of Pydantic. A good starting point for what it can do can be found here. The work-in-progress implementation on top of Pydantic's BaseModel class can be seen here.

The title of that project, pydantic-tests, gives an indication of my uncertainty that the idea would actually pan out; but in fact I now think it could improve things (from a maintenance and extension perspective) by leveraging an external framework. In code for personal/consulting Python projects I anticipate I will be using Pydantic quite a bit.