fix for issues 114 and 118: appended data paths for new regions

Adding new regions such as an APO state will require adding a path to Carmen.data_paths and putting a yaml file in that directory (this case us.yml). When you subsequently look up the regions for Canada, you will get the error.

```
Errno::ENOENT: No such file or directory - /custom_path/world/ca.yml
```

This commit makes sure the file exists before attempting to load its data. If the file doesn't exist (no overlay data) it just adds an empty array to the data_sets.