Create model harness + Mock model #18

Background

We need an interface for our models to interact with and should be based off the real model contract. This is also what our mock model needs to obey.

Description

For now, implement a class that defines an interface that takes in a grid and returns that model's outputs. How many outputs there are and what shape they take depends on the model, since the core model returns two grids and the decoder returns one, so the interface should not lock either of them into returning a single patch.

Then create a mock model that implements this interface and applies a deterministic function to the input grid and returns an output.