Changing the size of the rolling horizon

[senhalil]

Some countries have more reliable case number reporting and after the total number of cases gets big enough the rate stabilises itself and a week might be too pessimist for such countries (and might be hiding the effects of the measured taken).

A parameter to change the size of the rolling horizon would be interesting.

Thanks slot for this tool! It's ingenious.

[exclipy]

I saw @stofee implemented something like this in their branch at https://github.com/stofee/covidtrends/commits/master