Upcoming data structure changes in version 0.3.31

[RamiKrispin]

Upcoming changes in the data structure of the coronavirus data set - added the following fields:

• uid - Country code
• province_state - Province or state if applicable
• iso2 - Officially assigned country code identifiers with two-letter
• iso3 - Officially assigned country code identifiers with three-letter
• code3 - UN country code
• fips - Federal Information Processing Standards code that uniquely identifies counties within the USA
• combined_key - Country and province (if applicable)
• population - Country or province population
• continent_name - Continent name
• continent_code - Continent code