add factor potential example

Author: jsalvatier

examples/factor_potential.py

@@ -4,7 +4,7 @@
     x = Normal('x', 1,1)
     model.AddPotential(-x**2)
 
-    
-    h = find_hessian()
-    step = Metropolis(h)
-    trace = sample(3000, step)
+    start = model.test_point
+    h = find_hessian(start)
+    step = Metropolis(model.vars, h)
+    trace = sample(3000, step, start)