[WIP] Globe axis

[asinghvi17]

This PR builds towards a 3D Globe Axis, that currently will accept a Geodesy.jl datum as the ellipsoid definition, and try to work in ECEF coordinates. We can add more functionality, integration with CoordRefSystems.jl datums, etc later.

This axis will:

• Automatically apply a transform_func to all plots, going to 3D cartesian space.
• Lock certain zoom keys so you can interact with this like Google Earth
• Add a "satellite" camera-interaction API so you can move the camera in spherical space.
• Subsample polygon meshes so that they always lie on the sphere to some approximation, not under it
• Allow the following GlobeAxis-specific keywords:
  • radius_scale: scale the radius of the transformation, so it's easier to layer plots (e.g earth image below, polygons above, etc
  • source: same as GeoAxis source, indicate a source CRS
• Add a "background" sphere by default, using a modified icosphere algorithm, that could at some point be parameterized to hold some background image as well.

More todos to come. This is an expansion of the Geodesy.jl approach that was taken previously