For #463, properly handle duplicate comets in file

Turns out?  A given comet like `C/2020 F3 (NEOWISE)` can appear twice in
the Minor Planet Center’s `CometEls.txt`.  So let’s show users how to
select the most recent of several possible orbits.

examples/comet_neowise_chart.py

@@ -1,10 +1,8 @@
 import numpy as np
-import pandas as pd
 from matplotlib import pyplot as plt
 from matplotlib.collections import LineCollection
 
-from skyfield import api
-from skyfield.api import Star, load, tau
+from skyfield.api import Star, load
 from skyfield.constants import GM_SUN_Pitjeva_2005_km3_s2 as GM_SUN
 from skyfield.data import hipparcos, mpc, stellarium
 from skyfield.projections import build_stereographic_projection
@@ -28,7 +26,10 @@
 with load.open(mpc.COMET_URL) as f:
     comets = mpc.load_comets_dataframe(f)
 
-comets = comets.set_index('designation', drop=False)
+comets = (comets.sort_values('reference')
+          .groupby('designation', as_index=False).last()
+          .set_index('designation', drop=False))
+
 row = comets.loc['C/2020 F3 (NEOWISE)']
 comet = sun + mpc.comet_orbit(row, ts, GM_SUN)
 

skyfield/data/mpc.py

@@ -133,7 +133,7 @@ def d(s):
     'perihelion_distance_au', 'eccentricity', 'argument_of_perihelion_degrees',
     'longitude_of_ascending_node_degrees', 'inclination_degrees',
     'magnitude_H', 'magnitude_G',
-    'designation',
+    'designation', 'reference',
 )
 
 _fast_comet_re = None
@@ -166,7 +166,7 @@ def load_comets_dataframe(fobj):
                 pat.append(' ' * (start - previous_end))
             keep = name in keepers
             if name == 'designation':
-                pat.append('(.*?)  .*')
+                pat.append('(.*?)  +(.*)')
                 break
             else:
                 if keep:

skyfield/documentation/kepler-orbits.rst

@@ -61,8 +61,11 @@ but most Skyfield users will simply index their dataframe by comet designation.
 
 .. testcode::
 
-    # Index by designation for fast lookup.
-    comets = comets.set_index('designation', drop=False)
+    # Keep only the most recent orbit for each comet,
+    # and index by designation for fast lookup.
+    comets = (comets.sort_values('reference')
+              .groupby('designation', as_index=False).last()
+              .set_index('designation', drop=False))
 
     # Sample lookups.
     row = comets.loc['1P/Halley']