ISO-2 standard for country names

[DanielRobertNicoud]

All codes that take countries as input should take ISO-2 codes instead of the full name. All datasets should have a column 'country' with the full name and a column 'country_iso2' for the ISO-2 code.

1. Update the docs.
2. Update all codes.

[cgomez9]

@DanielRobertNicoud Is there anybody working on this task?

[ManuelAlvarezC]

This discussion should be done on Trello, @cgomez9.

More prioritary issues right now are:

• Finishing the Italian demographics datasource (https://trello.com/c/O84EQrAO)
• Fixing the us_census datasource: (Error 500 when calling us_census() #37) (Which by the way doesn't have an associated trello card, do you mind creating it and link the issue? Thanks)

[cgomez9]

Ok thank you Manuel, regarding the issue #37 the Trello card was already created (https://trello.com/c/AtA819Yd)