Add other statistical indicators

[KetsiaGuichard]

Similar to the proposal for visualizing average traffic, we could display other indicators: variance, standard deviation, median...
This could be done in two ways, depending on what is most relevant:

• Offer a 'summary statistics' function that calculates these indicators.
• Suggest visualizing the evolution of these indicators per hour.

Code modifications would be minor in the second case, but it would require generalizing the gg_traffic_avg function to work with other indicators.