Add social disctance dataset part 2

[ilovechai]

Description

OxCGRT collects publicly available information on 13 indicators of government response (S1-S13). Nine of the indicators (S1-S7, S12 and S13) take policies such as school closures, travel bans, etc. are recorded on an ordinal scale; the remainder (S8-S11) are financial indicators such as fiscal or monetary measures Also see: https://www.bsg.ox.ac.uk/research/research-projects/oxford-covid-19-government-response-tracker

Fixes: Added country_iso2 colmn as per #50, Added iso2 to iso3 map and iso3 to iso2 map as per #42, Added OxCGRT dataset for https://trello.com/c/V3JYaToE/88-social-distancing-policies

Checklist:

• [x] I have read the CONTRIBUTING GUIDE and made sure this Pull Request is compliant with it.
• [x] I have read the DATA MODEL and DATA SOURCES and made sure this Pull Request is compliant with it.
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules

[ilovechai]

@ManuelAlvarezC Made most of the changes. Please review and let me know what else needs to be changed.

• For the country_codes.py,
  • I created a dataframe and then a csv file
  • Copy pasted your translator function and tested if it worked : It worked fine
  • There are a few Countries for which iso2 and iso3 was not found. List is as below Europa Island
    Jan Mayen
    Johnston Atoll
    Juan De Nova Island
    Palmyra Atoll
    Midway Islands
    Tromelin Island
    Virgin Islands
    Wake Island

[hyberson]

Fix us_census datasource