Adjust get_body to take into account location on Earth #10271

In #10230 (comment), @mkbrewer noted that our solar_system._get_apparent_body_position, which is used in get_body, corrects for travel time to the center of the Earth rather than the actual location of the observer. While in general the resulting differences of a ms is not going to affect any of the locations of the planets at a level one likely cares about, the actual travel time is iterated on until it is less than 10 ns, which is suggesting better precision. And the docstring for get_body also seems to suggest the actual location of the observer is used. cc @StuartLittlefair

Also, crossref #10230 (comment), in which @ayshih mentions a sunpy routine that calculates light-travel times for an arbitrary observer.