1. Add `Sentinel-2`, `Blue Marble`, and `Earth at night` to BaseLayerPicker
2. Removed old Black Marble from the BaseLayerPicker.
3. Add new `ion Assets` tab to Sandcastle to showcase initial assets
available to Cesium users.
4. Update ArcticDEM example to use ion version instead of deprecated
assets.agi.com.
5. Rename PATerrain example to the more accurate `PAMAP Terrain` and have
it use ion version instead of deprecated assets.agi.com.
6. Add bare bones `Blue Marble`, `Cesium World Terrain`, `Earth at Night`,
`Natural Earth II`, `Sentinel-2`, and `Washington DC 2017` examples.
7. Update examples using deprecated cesiumjs.org version of Black Marble to
use the newer Earth at Night ion asset.
8. Resize some overly-large gallery images.
Matthew Amato