Add unit test for Mass multiple inheritance

The multiple inheritance property of Mass has been identified problematic in a previous pull request #325.

The test in this commit succeeds if Mass is defined with the label py::multiple_inheritance(). Otherwise it crashes.

Author: alxbilger

bindings/Sofa/tests/Core/Mass.py

@@ -54,3 +54,8 @@ def test_mass_matrix_access_blocks3x3(self):
         self.assertEqual(M.ndim, 2)
         self.assertEqual(M.shape, (960, 960))
         self.assertEqual(M.nnz, 9480)
+
+    def test_mass_mstate(self):
+
+        root = self.simulate_beam("CompressedRowSparseMatrixMat3x3d")
+        self.assertNotEqual(root.mass.mstate, None)