[bugfix] Change in the use of 'exec" within a function to make it works

also in Python3."

evolution_function.py

@@ -138,10 +138,11 @@ class UserDefined(EvolutionFunction):
 
     def f(self, X):
         """The actual performative function itself."""
-        exec(self.function_definition)  # define a function f
+        namespace = {}
+        exec(self.function_definition, namespace)  # define a function f
         Y = np.zeros(len(X))
         for i, x in enumerate(X):
-            Y[i] = f(x)
+            Y[i] = namespace['f'](x)
         return Y
 
     traits_view = View(