Deprecate a redundant method we never used

I was far too fearful of underlying libraries when I wrote Skyfield!  If
the user wants something low-level, like an ephemeris comments field, I
shouldn’t be afriad to tell them to drop down one level to look at it.
(Irony: `planets.rst` forgot this existed, and called the lower level!)

documentation/api.rst

@@ -182,7 +182,6 @@ See :doc:`planets`.
 
    SpiceKernel
    SpiceKernel.close
-   SpiceKernel.comments
    SpiceKernel.names
    SpiceKernel.decode
 

skyfield/jpllib.py

@@ -72,6 +72,7 @@ def __init__(self, path):
         self.segments = [SPICESegment(self, s) for s in self.spk.segments]
         self.codes = set(s.center for s in self.segments).union(
                          s.target for s in self.segments)
+        self.comments = self.spk.comments  # deprecated pass-through method
 
     def __repr__(self):
         return '<{0} {1!r}>'.format(type(self).__name__, self.path)
@@ -101,24 +102,6 @@ def close(self):
         del self.segments[:]
         self.codes.clear()
 
-    def comments(self):
-        """Return the comments string of this kernel.
-
-        The resulting string often contains embedded newlines, and is
-        formatted for a human reader.
-
-        >>> print(planets.comments())
-        ; de421.bsp LOG FILE
-        ;
-        ; Created 2008-02-12/11:33:34.00.
-        ...
-        LEAPSECONDS_FILE    = naif0007.tls
-        SPK_FILE            = de421.bsp
-        ...
-
-        """
-        return self.spk.comments()
-
     def names(self):
         """Return all target names that are valid with this kernel.