Added Python demo of calculating Jupiter's moons.

The demo shows how to correct for light travel
time to render Jupiter's moons as they appear
from the Earth.

Created an addition operator for the Vector
class in the Python code, because it is handy.

Corrected a bug in the string representation
of the Python StateVector class.

source/python/astronomy.py

@@ -162,6 +162,9 @@ class Vector:
         """Returns the length of the vector in AU."""
         return math.sqrt(self.x**2 + self.y**2 + self.z**2)
 
+    def __add__(self, other):
+        return Vector(self.x + other.x, self.y + other.y, self.z + other.z, self.t)
+
 class StateVector:
     """A combination of a position vector, a velocity vector, and a time.
 
@@ -197,7 +200,7 @@ class StateVector:
         self.t = t
 
     def __repr__(self):
-        return 'StateVector[pos=({}, {}, {}), vel=({}, {}, {}), t{}]'.format(
+        return 'StateVector[pos=({}, {}, {}), vel=({}, {}, {}), t={}]'.format(
             self.x, self.y, self.z,
             self.vx, self.vy, self.vz,
             str(self.t))