Optionally download models if they are not present

[Nelson-Gon]

In the current state, the app assumes that the models will always be present and does not test for this assumtion. This would break the app with no feedback to either the users or developers on what could possibly be wrong.

There are two possible ways to deal with this:

• Always check that the models load, and optionally download them or attempt a download if they do not exist
• Include the model checkpoints as part of the package and load them internally