Adds meta-test set evaluation script

[SMHendryx]

This PR adds a meta-test set evaluation script, which would be generally helpful for this repo and was requested here: #6

I refactored the model definition code to avoid code duplication.

Running this with your provided pre-trained model on the 5-shot setting produces

Mean IoU: 80.82 +/- 1.53

The published results of 80.12% fall within the confidence interval of these results above.

Please let me know if you have any questions or comments.

Thanks,
Sean