Add function to get the Earth-centered, Earth-fixed (ECEF) coordinates of the Sun

[krikru]

I want to get the Earth-centered, Earth-fixed (ECEF) coordinates of the Sun, but I can't find any function with which I can get those coordinates in the code today. I think this should be a quite simple function to implement, as it feels like a relatively natural coordinate system.

The ECEF coordinate system is a Cartesian coordinate system with the origin either at Earth's center of mass, or at the center of the reference ellipsoid (so there doesn't really seem to be a clear consensus, and which location to use for the origin probably mostly comes down to what is the most practical), and citing mathworks.com:

• The positive X-axis intersects the surface of the ellipsoid at 0° latitude and 0° longitude, where the equator meets the prime meridian.
• The positive Y-axis intersects the surface of the ellipsoid at 0° latitude and 90° longitude.
• The positive Z-axis intersects the surface of the ellipsoid at 90° latitude and 0° longitude, the North Pole.