Add missing property decorator

I have no idea why minimization has been working in the first place. The bug was found by trying to access WienerFilterEnergy(...).value.

nifty/library/wiener_filter/wiener_filter_energy.py

@@ -49,6 +49,7 @@ class WienerFilterEnergy(Energy):
     def curvature(self):
         return WienerFilterCurvature(R=self.R, N=self.N, S=self.S)
 
+    @property
     @memo
     def _Dx(self):
         return self.curvature(self.position)