split country attributes into separate data.frame and function

[alistaire47]

Country attributes that are not strictly country names or codes (tld, currency, capital, independence and territorial status (#2), etc.) should be split out of countries and accessed with a more apt function, maybe as_country_attr.

Reverse conversion (where sensible, e.g. tld or currency code) should be enabled through separate helpers to as_country_code and as_country_name akin to convert_country in order to avoid losing functionality. Conversion can probably use iso2c as a key to bridge the datasets. There are some observations without one, though once a list of attributes is collected, it will have to be seen whether they have any worth using a separate key column.

TODO:

• Assemble list of attributes to separate
• Check if all with data have iso2c to use as key column
• Rebuild aggregate.Rmd as necessary to generate separate data.frames
• Build separate parser—reuse convert_country?
• Add separate helper functions to as_country_code and as_country_name to allow back-conversion
  • Add warning for non-unique types or limit reverse conversion