Add module loading Global Human Settlement Layer (GHSL) data as exposure

[peanutfun]

The Global Human Settlement Layer (GHSL) provides information on human settlements on the earth. Most importantly for Climada, it provides a grid of population information in 100m resolution for 5-year epochs since 1975. We could create a module similar to LitPop, where the GHSL data is downloaded automatically and the Exposure information can be requested via functions similar to LitPop.from_countries.

This would be a functionality also interesting to MeteoSwiss @ThomasRoosli