Now descent minimizer and bugfix

nifty/energies/energy.py

@@ -39,10 +39,6 @@ class Energy(Loggable, object):
     def position(self):
         return self._position
 
-    @position.setter
-    def position(self, position):
-        self._position = position
-
     @property
     def value(self):
         raise NotImplementedError