[Tests] add initialize euler integrator test

Which segfault until
https://github.com/stack-of-tasks/dynamic-graph-python/pull/40

ref https://github.com/stack-of-tasks/dynamic-graph-python/issues/39

unitTesting/python/CMakeLists.txt

 SET(${PROJECT_NAME}_PYTHON_TESTS
   op-point-modifier
+  initialize-euler
   )
 FOREACH(TEST ${${PROJECT_NAME}_PYTHON_TESTS})
   ADD_PYTHON_UNIT_TEST("py-${TEST}" "unitTesting/python/${TEST}.py")

unitTesting/python/initialize-euler.py

+#!/usr/bin/env python
+
+import unittest
+
+import dynamic_graph as dgpy
+import dynamic_graph.sot.core.integrator_euler as ie
+
+
+class OpPointModifierTest(unittest.TestCase):
+    def test_initialize_ie(self):
+        ent = ie.IntegratorEulerVectorDouble("ie")
+        with self.assertRaises(dgpy.dgpyError) as cm:
+            ent.initialize()
+        self.assertEqual(str(cm.exception),
+                         'In SignalPtr: SIN ptr not set. (in signal <sotIntegratorAbstract(ie)::input(vector)::sin>)')
+
+
+if __name__ == '__main__':
+    unittest.main()