Include a default imagery layer for Cesium

[robyngit]

In order to use Cesium maps where we use Google maps, we should include a default imagery layer in Cesium maps that doesn't require Cesium Ion or another service. We could 'ship' this default layer with MetacatUI.

[robyngit]

If no layers are configured for a new Cesium map, then the Natural Earth II base layer will now be displayed (in branch feature-#1720-cesium-data-catalog):

The Natural Earth II layer can now also be added to any map by adding a layer with the following config to the layers attribute of a cesium map config: { type: 'NaturalEarthII' }

The images for this layer are shipped with Cesium and thus also shipped with MetacatUI

[robyngit]

We never merged the feature-#1720-cesium-data-catalog branch into develop or main because the Natural Earth Imagery is quite low resolution and not an ideal default layer. However, I think that this still makes for a reasonable fallback when there are no other options (i.e. it's better than rendering a big blue sphere with no imagery), so I would like to include it as an option.

@jeanetteclark recently discovered open imagery that is hosted by USGS. I'm re-opening this issue to cover adding a USGS layer as another default option.

[robyngit]

The USGS layer can now be configured by setting a layer asset to:

layers: [
  {
    type: 'USGSImageryTopo'
  },
  ...
]

The cesium imagery model takes care of setting the other required options (url, attribution, etc).

This is similar to how the Natural Earth layer can be configured:

layers: [
  {
    type: 'NaturalEarthII'
  },
  ...
]

Here is how the USGS layer looks from a global view:

It's higher resolution than the Natural Earth layer. Here's a random area in Alaska at around 100m scale:

The downsides are that the imagery disappears at around the 50-100m scale mark, and that there is no imagery covering the poles: