Find replacement API for covidtracking.com

[steven-cd]

In covid-policy-modelling/covid-policy-modelling#20, @sdwfrost noted the covidtracking.com API is scheduled to stop collecting data in March, and to shut down entirely in May. If we want to continue doing US simulations, we will need to find a replacement.

The possibilities that have been mentioned include:

• Our World In Data, which gets case & death info from JHU, although some other data from covidtracking.com, so may face similar issues
• ECDC, which we're already using (and who get US data from cdc.gov)

[kavousan]

The covidtracking.com API stops updating on the 7th March. This was originally used because the ECDC data did not provide US states data. The Covid tracking API became an API to the Johns Hopkins data - they used to collect the data themselves and do some munging themselves just as the Oxford tracking group still do; expensive job. ECDC does not host State-level data, but may suffice in the first instance and may not give us problems in the long run.