Check if mu is zero in OsculatingElements.__init__() (#259)

Raises a ValueError instead of having a mysterious numpy divide by zero warning later on
Be more specific and address the case of negative mu_km_s

skyfield/elementslib.py

@@ -53,6 +53,10 @@ class OsculatingElements(object):
 
     """
     def __init__(self, position, velocity, time, mu_km_s):
+        if mu_km_s <= 0: 
+            raise ValueError('`mu_km_s` (the standard gravitational parameter '
+                             'in km^3/s^2) must be positive and non-zero')
+        
         self._pos_vec = position.km
         self._vel_vec = velocity.km_per_s
         self.time = time