Add % Positivity (as per State) for input of model training + model fitting

Retrieve data from here: https://covidtracking.com/data/api

Add the following fields to data input:
* N Tested in State
* N Tested in US
* N Positive in State
* N Positive in US

**Consider transforming these metrics to a single risk score (relative % likelihood of covid * relative N positive of covid)?**