fix "outputs" argument of execute_graph

src/ewoksppf/bindings.py

src/ewoksppf/tests/test_examples.py

src/ewoksppf/tests/test_ppf_end.py

src/ewoksppf/tests/test_ppf_workflow1.py

src/ewoksppf/tests/test_ppf_workflow10.py

src/ewoksppf/tests/test_ppf_workflow11.py

src/ewoksppf/tests/test_ppf_workflow12.py

src/ewoksppf/tests/test_ppf_workflow13.py

src/ewoksppf/tests/test_ppf_workflow14.py

src/ewoksppf/tests/test_ppf_workflow15.py

src/ewoksppf/tests/test_ppf_workflow16.py

src/ewoksppf/tests/test_ppf_workflow17.py

src/ewoksppf/tests/test_ppf_workflow18.py

src/ewoksppf/tests/test_ppf_workflow19.py

src/ewoksppf/tests/test_ppf_workflow2.py

src/ewoksppf/tests/test_ppf_workflow20.py

src/ewoksppf/tests/test_ppf_workflow21.py

src/ewoksppf/tests/test_ppf_workflow22.py

src/ewoksppf/tests/test_ppf_workflow23.py

src/ewoksppf/tests/test_ppf_workflow24.py