Implement a Model repository for the data access layer

[hbooth]

Use the repository pattern to create a repository for the Model Resource. This will be used to refactor parts of the Model service layer.

This should encompass calls to the mlflow tracking client. It should be injected into the repository class so that it can be swapped out with a mocked client for testing.

Blocked by:

• Update Model service layer to register the model in MLFlow #747
• Update Model service layer to call MLFlow model version registration #748

Definition of Done:

• The Model repository is implemented
• The Model service layer is refactored to make use of the Model repository
• All existing tests pass
• The feature is merged into dev