Improve map

[damianooldoni]

This PR:

• Improves tile backgrounds as described by @biekemaex in Show reserved locations on the map #210 (comment)
• Use color palette as described by @biekemaex in Show reserved locations on the map #210 (comment). Notice I increased the saturation of the circles otherwise the colors on the map were not always similar to the real colors in the legend.
• Reads locations following the specs as described by @fjasteen in Show reserved locations on the map #210 (comment)
• Add absence layer called "Null waarnemingen". At the moment I use the same circles as presences. I will visualize them as markers (see Show absences #271) in a separate PR later