Update Observer.at_site to use astropy site machinery

[eteq]

astropy/astropy#4042 is now merged, so it probably makes sense to switch to that in favor of the astroplan get_site machinery.

To clarify one additional thing about this: I think it may make sense for the data.astropy.org/coordinates/sites.json file to get additional information down the road about sites. That info may be used by astroplan even if its ignored by astropy itself. So no need to worry that it's now "out of astroplan's control": astropy-data is an easy thing to change! I don't think that's particularly relevant right now, but it might be if more information gets added to Observer that's not in EarthLocation but should be in the data file.

[eteq]

Also, I believe @bmorris3 volunteered to do this in astropy/astropy#4042, so I'll assign him.