Add example notebook on extending Gumbi by inheriting `Regressor`

[JohnGoertz]

The core functionality of Gumbi is contained in the Abstract Base Class Regressor. This was written to allow simple extension with custom models and inferrence methods through defining the abstract methods fit, build_model, and predict. There should be a notebook demonstrating how this could be achieved, potentially with a Bayesian Neural Network or a Generalized Linear Model.