Update OrbitFG.py to fix bug in issue 2

Not yet tested...

Author: cntaylor

OrbitFG.py

@@ -159,7 +159,7 @@ def F_mat(self,state):
             dist = la.norm(curr_state[:2])
             k = self.prop_dt*self.GE/(dist**5)
 
-            t_mat = np.array([[-self.prop_dt*(y**2-2*x**2)/2, 3*x*y*self.prop_dt, 0, 0],
+            t_mat = np.array([[-self.prop_dt*(y**2-2*x**2)/2, 3*x*y*self.prop_dt/2.0, 0, 0],
                               [3*x*y*self.prop_dt/2.0, -self.prop_dt*(x**2-2*y**2)/2, 0, 0],
                               [2*x**2-y**2, 3*x*y, 0, 0],
                               [3*x*y, 2*y**2-x**2, 0, 0]])