Adding solar system targets to astroplan

[hhsieh00]

As part of work to develop sbpy (for small solar system bodies), we are also interested in adding solar system capabilities to other packages (e.g., astroquery) where it makes sense to do so vs. duplicating existing infrastructure, and one item on our list is to add solar system objects to astroplan. I see that PR #63 relates to this, but it seems the last work on that was 10 years ago. Before starting any significant new work on this, I just wanted to check in (particularly with @bmorris3 who opened that old PR) to see if there's been any other progress since 2015 (or alternatively, any intentions to resume work on it in the near future), and if we start working on this again, if it would be okay if we started a new PR.

Basically the idea would be to use astroquery's JPL Horizons query functionality (instead of pyephem) to return object coordinates at certain time intervals, and then figure out how to integrate those lists of coordinates into astroplan's plotting tools.

Any comments or additional background info I might be missing would be greatly appreciated.

Thanks!
Henry