Use more robust coordinates machinery in get_skycoord

This is basically a move of this comment: https://github.com/astropy/astroplan/issues/382#issuecomment-441892022, which turns out to be separate from the "real" issue #382 . @adrn pointed out there that the `get_skycoord` function could probably be updated to use some of the astropy machinery that concatenates sky coordinates in a way that's probably more likely to remain stable/without warnings since it would defer more of the "hard work" to astropy itself.

Start by locating the get_skycoord function and read the discussion moved from issue #382, especially the referenced comment. Investigate the Astropy machinery for concatenating sky coordinates and compare it with the current approach. Done means get_skycoord uses the more stable Astropy path without introducing warnings or changing expected coordinate behavior.

Written by the indexing model from the issue text.