Resolves subregion error when including additional data paths

This addresses an error with subregions when adding another `data_path` to Carmen. The library expected there to be subregion files in the added `data_path` for *all* of the regions specified in the gem's included `world.yml` file, and thusly would throw an error when trying to load data from the non-existent YAML files, not allowing you to even access existing subregions.

I believe the specs are reasonable, but if you have any questions or suggestions just let me know. :)