test for XDMF writer

just ensure that it does not throw an exception

python/tests/reference/Result/6grains6x7x8_single_phase.geom

@@ -0,0 +1,61 @@
+4 header
+grid   a 6 b 7 c 8
+size   x 0.75 y 0.875 z 1.0
+origin x 0.0 y 0.0 z 0.0
+homogenization 1
+3 3 3 4 3 3
+3 1 1 1 3 3
+3 5 1 1 1 3
+1 5 5 1 1 1
+1 5 5 1 1 1
+6 3 3 4 1 6
+6 3 3 4 4 6
+6 3 3 1 3 3
+3 1 1 1 3 3
+3 1 1 1 1 1
+1 1 1 1 1 1
+6 6 3 1 1 1
+6 3 3 3 6 6
+6 3 3 3 6 6
+6 3 3 1 1 6
+3 1 1 1 1 3
+6 1 1 1 2 2
+1 6 2 2 2 2
+6 6 2 2 2 6
+6 3 3 3 6 6
+6 3 3 3 6 6
+5 6 6 6 1 6
+6 6 6 6 2 2
+6 6 6 2 2 2
+2 6 2 2 2 2
+6 5 2 2 2 2
+6 5 5 2 2 6
+5 5 5 3 6 6
+5 5 6 6 6 5
+6 6 6 6 6 6
+6 6 6 6 2 2
+4 4 6 2 2 2
+4 4 2 2 2 2
+5 5 5 2 2 2
+5 5 5 5 2 5
+5 5 5 4 4 5
+6 6 6 6 4 4
+4 4 5 5 2 4
+4 4 5 2 2 4
+4 4 2 2 2 2
+5 5 5 2 2 2
+5 5 5 4 4 5
+5 5 4 4 4 3
+4 5 5 5 4 3
+4 4 5 5 5 4
+4 4 5 5 2 4
+4 4 2 2 2 2
+5 5 2 2 2 2
+5 5 4 4 4 4
+3 4 4 4 4 3
+3 5 5 4 3 3
+4 5 5 5 3 3
+4 5 5 5 1 1
+4 4 5 2 1 1
+6 4 4 4 4 1
+3 4 4 4 4 3

python/tests/test_Result.py

@@ -20,6 +20,13 @@ def default(tmp_path,reference_dir):
     f.pick('times',20.0)
     return f
 
+@pytest.fixture
+def single_phase(tmp_path,reference_dir):
+    """Single phase Result file in temp location for modification."""
+    fname = '6grains6x7x8_single_phase_tensionY.hdf5'
+    shutil.copy(os.path.join(reference_dir,fname),tmp_path)
+    return Result(os.path.join(tmp_path,fname))
+
 @pytest.fixture
 def reference_dir(reference_dir_base):
     """Directory containing reference results."""
@@ -296,3 +303,7 @@ class TestResult:
     def test_vtk(self,tmp_path,default,output):
         os.chdir(tmp_path)
         default.to_vtk(output)
+
+    def test_XDMF(self,tmp_path,single_phase):
+        os.chdir(tmp_path)
+        single_phase.write_XDMF