Outputting the intervention and country data from plot-3-panel.r required to generate the plots to csvs

[hrkulkarMsft]

This branch adds code to plot-3-panel.r to output:

1. Intervention data
2. Data for all countries grouped into a single CSV

This simplifies the downstream consumption of this data into visuals for the PowerBI for now.

Code Changes

The code changes here are to:

• Accumulate each country's data into a single data frame for any data required for the 3-panel graph
• Output said data to a csv file

Behavior Change

The above code changes now output two new flat csv files to the "results/" folder:

• base-plot.csv : containing data for all countries required for the 3-panel plot.
• base-intervention.csv: containing data required to plot the interventions on the 3-panel plot.

Other

@fvalka should also review this, as these outputs should be a part of the batch process.