C19 NN Update World USA Arkansas - new look

Some new look additions to the plots

1. As always, the cumulative C19 death data is randomly split into test and train groups. now distinguished by color on the top plot.
2. US cumulative plot now indicates C19 federal relief measures signed into law as red triangles, and seasonal flu deaths as dashed black line

Note Arkansas has a new record of 20 deaths in one day.