added china by provinces

[elsaburren]

based on jhuCSSE. So I just took MSZ.R and wrote China into it

[eguidotti]

Thanks for your pull request! Here my comments:

• China is already supported by the package. Infact, all data from jhuCSSE are already exported by the internal function WORLD(). You can check this by running covid19("CHN") (country-level) and covid19("CHN", 2) (state-level)
• you mention that you are adding China by province, but it seems to me that it is by country (level 1)
• the function is missing the id field to include in the data frame before returning

Since China is already supported with JHU data I'm going to close this request. Please note that all the data sources implemented in the package are already exported to the end user at the moment. We are now committed in supporting new data sources. Then as a second step exporting the new data to the user.