**WARNING:** loading and processing can take a long time (30 sec on my computer)

What is it for ?
- to show how slow heatmap computation can be (there are 36000 cities in france)
- to show where population denser in France
- to show where the biggest cities are
- to show there is NO a single heatmap, the algorithm has many parameters you can tweak to produce a large family of maps
- Many cities can bring contribution to a single pixels. How these contribution are mixed together, Sum? max, mul?
- Every contribution from a city only affect a small circle around the position of the city. how big is this circle ? proportional to the population itself ? 5, log2 or log10
- Every contribution from a city decrease inside the circle when distance from the center increase (it is also proportional to the population). What distance is used ? 1, d, d*d or d*d*d ?