Deprecate the Proxy class.

[roads]

The Proxy class was originally created to insulate a user from TF dataset formatting and model restarts. As the project has evolved, this "wrapper" abstraction feels increasingly redundant and confusing. A cleaner approach would remove the Proxy class completely from the API and instead clearly document appropriate usage of the core PsychologicalEmbedding model.

One side-effect of removing the Proxy class is that that the Proxy class provides the only convenient method for computing the similarity between arbitrary points. This functionality would need to be provided elsewhere, perhaps in a standalone method.

[roads]

Proxy class has been deprecated and removed. Users should create a model (via subclassing or the functional API) to compute pairwise similarity.