Add elevation to the Cesium map scale bar view

[robyngit]

The current version of the scale bar shows the lon and lat position of the mouse:

When there is terrain data loaded, it would be nice to also show an estimate of elevation. However, Cesium's built in function for getting elevation at a point (scene.globe.pick) has issues, see the discussion here: CesiumGS/cesium#4368

Terria.JS has a nice solution for this, but it required that they extend Cesium to add a pickTriangle function to globe.