Extend current work to capture R-effective which help model trends, causal anal about covidtrends

[leander-j]

The option to select active instead of total cases as a reference is getting more relevant every day, since now that we are past the first wave and exponential rise, total cases are dominated by deaths and recoveries instead of active cases.

This is exactly were we need data about the effective reproduction to determine the current spread!

Thanks in advance!