normalize descent direction for SteepestDescent and VL_BFGS (but not for RelaxedNewton\!)

nifty/minimization/steepest_descent.py

@@ -39,4 +39,6 @@ class SteepestDescent(DescentMinimizer):
 
         """
 
-        return -energy.gradient
+        res = energy.gradient.copy()
+        res *= -1./res.norm()
+        return res

nifty/minimization/vl_bfgs.py

@@ -84,6 +84,7 @@ class VL_BFGS(DescentMinimizer):
         for i in xrange(1, len(delta)):
             descent_direction += delta[i] * b[i]
 
+        descent_direction *= 1./descent_direction.norm()
         return descent_direction