TLE Target

Hello everyone,

I'm exploring the possibility of scheduling observations for satellites defined by their TLE. I've dug into Astroplan, but it seems there's no current functionality in that area. I did come across an older [pull request that attempted an initial implementation of the "NonFixedTarget" object](https://github.com/astropy/astroplan/pull/63). It appears to be a generic approach where calculations for TLE targets could be facilitated through the `coord_function` argument. However, given the passage of time, I assume the integration with the rest of the code is likely outdated.

On the Astropy side, it seems that TLE coordinates aren't directly supported as a coordinate system, as [outlined here](https://docs.astropy.org/en/stable/coordinates/satellites.html). The recommendation appears to be to use another library, like [Skyfield](https://rhodesmill.org/skyfield/) or [SGP4](https://pypi.org/project/sgp4/), along with some custom code.

I'm curious to hear any thoughts on this matter. How could this be implemented? Are there more recent ideas related to the NonFixedTarget object?

Start by reviewing pull request #63 and the coord_function approach; the issue provides no file or test entry point. Compare the proposed NonFixedTarget design with the satellite-coordinate guidance and the possible Skyfield or SGP4 integration. Done means an agreed implementation direction and clarified scope for scheduling TLE-defined satellites.

Written by the indexing model from the issue text.