Document additional usage of closest_apparition

[mkelley]

astroquery's Horizons.ephemerides has a closest_apparition parameter that selects the orbit closest to the epoch(s) being requested. The default is to select the most recent orbit when closest_apparition is True. According to JPL Horizons's documentation other orbits can be selected:

     CAP;         (return last apparition before current date)
     CAP < JD#;   (return last apparition before specified Julian Day Number)
     CAP < YEAR;  (return last apparition before given integer year)

This is possible with astroquery, but undocumented. We should update astroquery's documentation and provide examples in sbpy's documentation.

>>> e1 = Ephem.from_horizons('90000083', epochs=Time('1975-01-01'))  # 90000083 is the record number for the 1974 orbit at the time of writing
>>> e2 = Ephem.from_horizons('2P', id_type='designation', epochs=Time('1975-01-01'), closest_apparition='<1975')
>>> e1['ra'] - e2['ra']                                                                                       
<Quantity [0.] deg>
>>> e1['dec'] - e2['dec']                                                                                     
<Quantity [0.] deg>

# e3 will be based on the 2015 orbit
>>> e3 = Ephem.from_horizons('2P', id_type='designation', epochs=Time('1975-01-01'), closest_apparition=True)
>>> e1['ra'] - e3['ra']
<Quantity [0.01268] deg>
>>> e1['dec'] - e3['dec']
<Quantity [0.00359] deg>