gdp: https://raw.githubusercontent.com/datasets/gdp/master/datapackage.json
country-codes: https://raw.githubusercontent.com/datasets/country-codes/master/datapackage.json
population-density: https://raw.githubusercontent.com/datasets/population/master/datapackage.json

Add list of external data packages
Add list of external data packages

This will allow for testing of in-development functionality to download
and use external data packages.
