Map view

Using https://www.react-simple-maps.io/:

• Add a fourth view called 'Map' (in MultiViewWidget.scala)
• Pan-zoomable (https://www.react-simple-maps.io/examples/zoom-pan/)
• Tooltip (https://www.react-simple-maps.io/examples/map-chart-with-tooltip/)
• Countries/World regions are gradient-colored proportionally to some numeric property (https://www.react-simple-maps.io/examples/world-choropleth-mapchart/). The property can be count or any numeric attribute (so exactly the same as graph, but without the concept of cumulative sum). Will need to introduce an extra attribute type geo and add support for multiple types (since countries will be both cat and geo).