Add support for ellipsoid-based voxels

[j9liu]

Description

Depends on #1685, so merge that first.

Data provided by NCEI

This PR adds support for loading and rendering ellipsoid-based voxels. Unlike box voxels, ellipsoid-based voxels account for the curvature of the Earth, and are thus effective at displaying data across a large region on the globe.

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated the documentation as necessary.

Testing plan

Reach out to me offline for the pictured test data.

Known Issues

There are various visual artifacts... you may have noticed the dithering artifacts along the edges of the data, as well as the hole in the middle. There is also a purple stripe that might be the result of float imprecision with the shape math:

These artifacts appear in CesiumJS as well, but they are hidden by either depth testing or linear interpolation.