Terrain/elevation

[rmcarder]

We should change the basemap to terrain, and look into doing something like this that shows inundation zones at a specified elevation

https://www.mapbox.com/bites/00307/?elev=100#13/37.9462/-122.5558

[mikelmaron]

Damon added mouse-movement listening / terrain querying into a modification of the above demo, here: https://bl.ocks.org/dnomadb/raw/56480aee163baa28fa0de92a0c02aa60/?elev=100#11/37.4591/-122.2202

(Right now, just logs out the elevation value)

Caveat(s): originally wrote this all a number of years ago, so could stylistically use an upgrade. In addition, it uses web workers to offload the elevation work to a separate thread which adds a good deal of complexity to the demo.