Addition of ISO Country Codes (Char and Num) to gapminder data objects

[jrebane]

Description

This commits changes the following:

• Adds ISO 3166-1 alpha-3 and ISO 3166-1 numeric country codes as two additional variables for the gapminder and gapminder_unfiltered objects
• Updates documentation (mostly) to reflect added variables

Motivation and Context

• Addresses ISO 3166-1 country codes #13

How Has This Been Tested?

• Local tests have not been run yet.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests pass

[jennybc]

Thanks! I took your code for getting these codes and reworked it in the poorly-named commit 700b427 and subsequent commits. I tried to bring this branch local and find a way to merge it, but there was too much conflict. I didn't want to bring the country codes into the main data frame. I've credited you in NEWS.

Thanks again.