FIX: Use class attribute instead of outer variable in PET test

Use class attribute instead of outer variable in PET test dummy class.

Author: jhlegarreta

test/test_integration_pet.py

@@ -76,7 +76,7 @@ def __init__(self, dataset, timepoints, xlim):
         def fit_predict(self, index):
             if index is None:
                 return None
-            return np.zeros(ds.shape3d, dtype=np.float32)
+            return np.zeros(self.dataset.shape3d, dtype=np.float32)
 
     monkeypatch.setattr("nifreeze.estimator.PETModel", DummyModel)