Add new dataset: GHSL

[darribas]

It'd be really cool to add an additional raster with non-traditional raster info. I'm thinking the GHSL for population. This would fit well in:

• Ch.3 (CHAPTER: Spatial Data Processing #23): manipulating objects as surfaces
• Ch.4 (CHAPTER: Weights #26) : weights from rasters
• Ch.7 (CHAPTER: Local Spatial Autocorrelation #25): local statistics from a raster (extension since chapter is short)

I'm happy to add it myself. As for regions, I was thinking of using somewhere in Latin America, perhaps Sao Paulo (Brazil)?

[darribas]

The data is ready:

https://github.com/gdsbook/book/tree/master/data/ghsl

It needs linking on the datasets list on the menu side bar

[darribas]

The dataset is merged and display on the website as of #164. Closing for now as the bare minimum is achieved. We may reopen later if we feel there's space to add the bits on Ch.4 (#26) and Ch. 7 (#25)